OPINION
DOGGETT, Justice.We consider whether an individual should be barred from testifying as a fact witness in his own defense because he in*90completely answered an adversary’s interrogatory seeking discovery of potential witnesses. Under the circumstances of this case, we hold that his testimony should not be excluded.
Southwest Feed Yards, Ltd. sued Arnold T. Smith to recover upon an open account for custom feeding of his cattle. Smith personally answered interrogatories propounded to him by Southwest but failed to include his own name in response to one of these seeking disclosure of potential witnesses. In compliance with the pre-trial order, he did give notice of his intent to testify as a witness seven days before trial. When he later attempted to testify, Southwest objected on the ground that he was not listed among those named in response to its interrogatory seeking the identity of persons with knowledge of relevant facts. After sustaining this objection and denying Smith an opportunity to testify, the trial court entered judgment based upon a jury verdict for Southwest. The court of appeals affirmed. 811 S.W.2d 717.
In our system of justice, discovery plays a vital role to assure “that disputes [are] decided by what the facts reveal, not by what facts are concealed.” Jampole v. Touchy, 673 S.W.2d 569, 573 (Tex.1984). A trial should be based upon the merits of the parties’ claims and defenses rather than on an advantage obtained by one side through a surprise attack. Rule 166b(2)(d) of the Texas Rules of Civil Procedure specifically authorizes discovery “of any potential party and of persons having knowledge of relevant facts.” Rule 215(5) mandates exclusion “of trial testimony of any [undisclosed] person having knowledge of discoverable matter, unless the trial court finds that good cause sufficient to require admission exist.” In all eleven of the cases in which this court has considered the propriety of excluding witnesses not identified in answer to an appropriate interrogatory, we have determined that “good cause” to permit testimony did not exist.1 In none of these decisions, however, was the excluded testimony that of an individual who was himself a party.
By filing a pleading, a party indicates at least a potential awareness of facts that bear on the merits of a claim or defense. We do not, however, accept Smith’s contention that this distinction alone is decisive. Excluding every party from the identification authorized by Rule 166b(2)(d) would open a broad loophole encompassing every employee of an entity and every plaintiff in a large class action.2 A party cannot disregard procedural rules and still insist upon an absolute right to testify in all circumstances. The importance attached to a party’s ability to testify in his or her own behalf constitutes only an additional factor that a trial court must consider in making its good cause determination.
In initiating its action here, Southwest pleaded that Smith was an individual indebted to it. Among the nine interrogatories he answered thirteen months before trial were the following:
INTERROGATORY NO. 4: Describe in detail any conversations you have had with the Plaintiff or plaintiff’s representative concerning this account.
ANSWER: 1. During October, 1988, I asked Wayne Chastain to discuss the account to straighten out the bill.
2. The next conference was at lunch with Wayne Chastain after *91receipt of demand letter. The conversation concerned my position which was well received. Mr. Chastain agreed to check out my explanation of the account.
INTERROGATORY NO. 5: State the name and address of each person, including experts, having any knowledge of relevant facts related to the account which is the basis of this suit. ANSWER: 1. Bookkeeper, Pam McCormick, Hereford, Texas.
2. Manager, Wayne Chas-tain, Canyon, Texas.
Although he certainly should have included himself among those persons listed in response to No. 5, Smith provided an answer to No. 4 that demonstrated his personal knowledge of facts relevant to this lawsuit. In determining whether “good cause” exists to permit his testimony, the substance of his entire response should be considered, not just his incomplete reply to a single query.
Our consistent construction of “good cause” for purposes of Rule 215(5) has been designed to prevent trial by ambush, not to create a trap for the unwary. In no way has Southwest been ambushed. We believe that the substance of Smith’s response to Southwest’s discovery identified him as a potential trial witness. Accordingly, we hold that the trial court abused its discretion in denying him an opportunity to speak in his own defense.
In reaching this conclusion, we recognize the need for caution to avoid a loose interpretation of “good cause” under Rule 215(5) that would defeat the objective of full disclosure of potential witnesses sought in our prior opinions. Given the complexity of much modern discovery, Rule 166b(2)(d) can usually accomplish its objective only if all potential witnesses are fully identified in response to the standard “persons with knowledge of relevant facts” interrogatory. Permitting litigants to summon parties not so identified on the grounds that they were one among many deponents or had been indirectly referenced in some other interrogatory answer would only serve to reintroduce trial by ambush. It is for that reason that we said in Sharp v. Broadway National Bank, 784 S.W.2d 669, 671 (Tex.1990), that
A party is entitled to prepare for trial assured that a witness will not be called because opposing counsel has not identified him or her in response to a proper interrogatory. Thus, even the fact that a witness has been fully deposed ... is not enough to show good cause for admitting the evidence where the witness was not identified in response to discovery.
This conclusion was reaffirmed in Alvarado v. Farah Mfg. Co., 830 S.W.2d 911, 915 (Tex.1992).
Under the circumstances of the instant case, where the answer to another query in a single, short set of interrogatories plainly indicated that the individual responding had knowledge of relevant facts, the trial court abused its discretion by failing to find “good cause” to permit that party’s testimony. In defining the scope of that discretion, we recognize the difficulties posed to a trial court in determining “good cause.” Certainly a more obscure or indirect identification than that involved here could permit exclusion of a party witness. Additionally, the constraints of Rule 215(5) may permit testimony by a party who is an individual not listed in response to a Rule 166b(2)(d) interrogatory, when identity is certain and when his or her personal knowledge of relevant facts has been communicated to all other parties, through pleadings by name and response to other discovery at least thirty (30) days in advance of trial. Cf. Exocet Inc. v. Cordes, 815 S.W.2d 350, 352-54 (Tex.App.-Austin 1991, no writ) (within trial court discretion to permit testimony by individual party not listed in response to interrogatory that was misleading and failed to conform with Rule 166(2)(d)).
In broadening the applicable considerations for determining good cause, we do not, as the dissent asserts, retreat in our commitment to fair discovery nor weaken the ability of the trial courts to sanction discovery abuse. Rather, we accord to the *92trial court some discretion, when the ability of an individual party to testify is disputed, in deciding whether good cause has been shown.3
Pursuant to Tex.R.App.P. 170, we grant Smith’s application for writ of error and without hearing oral argument, the judgment of the court of appeals is reversed and this cause is remanded to the trial court for a new trial consistent with this opinion.
GONZALEZ and HECHT, JJ., concur with concurring opinions. CORNYN, J., dissents with opinion.. Alvarado v. Farah Mfg. Co., Inc., 830 S.W.2d 911 (Tex. 1992); Sharp v. Broadway Natl Bank, 784 S.W.2d 669 (Tex. 1990); Rainbo Baking Co. v. Stafford, 787 S.W.2d 41 (Tex.1990); Gee v. Liberty Mut. Fire Ins. Co., 765 S.W.2d 394 (Tex. 1989); McKinney v. National Union Fire Ins. Co., 772 S.W.2d 72 (Tex.1989); Clark v. Trailways, Inc., 774 S.W.2d 644 (Tex.1989) cert. denied, 493 U.S. 1074, 110 S.Ct. 1122, 107 L.Ed.2d 1028 (1990); Boothe v. Hausler, 766 S.W.2d 788 (Tex. 1989); E.F. Hutton & Co. v. Youngblood, 741 S.W.2d 363 (Tex.1987); Gutierrez v. Dallas In-dep. Sch. Dist., 729 S.W.2d 691 (Tex.1987); Morrow v. H.E.B., Inc., 714 S.W.2d 297 (Tex.1986); Yeldell v. Holiday Hills Retirement and Nursing Ctr., Inc., 701 S.W.2d 243 (Tex.1985).
. Sharing the dissent’s concern that the exception we create today not undermine the purpose of the rule — to identify persons with knowledge of relevant facts sufficiently in advance of trial to permit discovery — today’s holding does not extend to situations in which identity is not easily ascertainable.
. Because our holding is narrow, the concern raised by the dissent that " 'the exceptions will consume the rule’ ” is significantly lessened. At 95 (quoting Paul N. Gold, Discovery Update: Reports From the Front, in State Bar of Texas Prof. Dev. Program, Litigation Update J-l, J-16 (1992)). Indeed, we have today chosen the same course advocated by the commentator upon which the dissent relies:
[T]o restrict the trial courts’ flexibility to pragmatically mold the rules to apply to specific circumstances raises the disdained situation of having procedural form dictate over sub-
stance.... If it is feasible, the practice of designating individuals with knowledge of relevant facts might benefit from the establishment of some broader boundaries for court discretion. In this connection, the best place to focus attention would be on the applicable criteria for determining good cause.
Gold at J-16 (emphasis added).