delivered the opinion of the Court,
in which Chief Justice PHILLIPS, Justice GONZALEZ, Justice HIGHTOWER, Justice HECHT, Justice GAMMAGE, and Justice ENOCH join.We consider trial court exclusion of the testimony of an expert witness on the grounds that he had not been designated “as soon as practical” under Rule 166b(6)(b) of the Texas Rules of Civil Procedure.
While crossing a street, Thelma Mentis was struck and killed by an automobile driven by John Barnard. Although suit was filed shortly after her death, the attorney who tried the case was not retained until about three months before trial. In response to discovery, that attorney designated an accident reconstruction expert as a testifying witness thirty-two days before trial. After jury selection and opening statements had been completed, Barnard’s attorney was successful in having barred any testimony from this expert because he had not been identified “as soon as practical.” Based upon the remaining evidence it was permitted to consider, the jury found that the negligence of both Mentis and Barnard constituted a proximate cause of the accident, with 85 percent of the negligence attributable to the former, and 15 percent to the latter. The trial court entered a take-nothing judgment. The court of appeals affirmed. 853 S.W.2d 119. We reverse.
Rule 166b(6)(b) addresses a party’s duty to supplement a discovery request regarding the identity or subject matter of an expert witness. Supplementation must occur “as soon as is practical, but in no event less than thirty (30) days prior to the beginning of trial except on leave of court.” TexR.Civ.P. lOOblOXb).1 A witness not timely designated is automatically excluded from testifying unless good cause is shown. TexR.Civ.P. 215(5); see, e.g., Gee v. Liberty Mut. Fire Ins. Co., 765 S.W.2d 394 (Tex.1989); E.F. Hutton & Co. v. Youngblood, 741 S.W.2d 363 (Tex.1987); Morrow v. H.E.B., Inc., 714 S.W.2d 297 (Tex.1986) (per curiam).
Whether any designation has been made or whether such designation occurred less than 30 days before trial is easily determinable; however, the meaning of “as soon as practical” is less definite. In the latter situation, which is presented here, the trial court need consider good cause for late identification only if it finds that the witness was not designated as soon as was practical.
Some courts appear to have read the “as soon as practical” requirement completely out of Rule 166b(6)(b) so long as the witness was designated more than thirty days before trial. Pedraza v. Peters, 826 S.W.2d 741 (Tex.App.—Houston [14th Dist.] 1992, no writ); Mother Frances Hosp. v. Coats, 796 S.W.2d 566 (Tex.App.—Tyler 1990, orig. proceeding). Other courts have applied a more stringent standard:
By adding the language “as soon as practical” the Texas Supreme Court has clothed the trial court with the discretion to suppress the testimony of an expert witness if his identity is tendered less than *16thirty days before trial commences or when the trial court finds the name was not submitted as soon as practical.
Builder’s Equip. Co. v. Onion, 713 S.W.2d 786, 788 (Tex.App.—San Antonio 1986, orig. proceeding) (per curiam). See also First Title Co. of Waco v. Garrett, 802 S.W.2d 254 (Tex.App.—Waco 1990), rev’d on other grounds, 860 S.W.2d 74 (1993); Williams v. Crier, 734 S.W.2d 190 (Tex.App.—Dallas 1987, orig. proceeding [leave denied]).
The plain wording of Rule 166b(6)(b) indicates that the duty to designate attaches only when “the party expects to call an expert witness [not] previously disclosed.” Tex R.Civ.P. 166b(6)(b). This provision does not establish a time after filing or answering a lawsuit by which a party must retain its testifying experts. See Mother Frances, 796 S.W.2d at 571. Neither does it require identification immediately upon contacting an expert for potential testimony. The rule does require the attorney to communicate the designation “as soon as practical” once it is decided that the expert is expected to be called at trial.
A trial court’s exclusion of an expert who has not been properly designated can be overturned only upon a finding of abuse of discretion. Morrow, 714 S.W.2d at 298. Barnard’s only objection was that “the designation of the [Mentis’s] expert one month before trial, after a lawsuit ha[d] been filed almost two years, is not as soon as practical.” Though later arguing that he lacked the expert’s deposition, Barnard’s counsel admitted that he had never sought it. A litigant who seeks to deny an opponent the right to use a witness has the burden of producing evidence to show that the designation was not “as soon as practical.” See Williams, 734 S.W.2d at 193. Simply advising the court as to how long the case had been pending did not by itself establish that designation at an earlier date would have been practical. The trial court abused its discretion in granting Barnard’s motion.
Our analysis is not complete, however, without determining whether the error constituted reversible error. TexRApp.P. 81(b). An error in the exclusion of evidence requires reversal if it is both controlling on a material issue and not cumulative. See Gee v. Liberty Mut. Fire Ins. Co., 766 S.W.2d 394, 396 (Tex.1989). Here the excluded witness was an accident reconstruction expert who planned to address a central issue of the comparative negligence of the pedestrian and driver. The expert’s noncumulative calculations and opinions, based on examination and measurement of the scene, that Barnard was driving between 6 and 20 miles above the 40 mile per hour speed limit2 on a dark, rainy morning were controlling, though not conclusive, as to whether excessive speed caused the death.
Given the harmful effect of the trial court’s abuse of discretion in excluding expert testimony, we reverse the court of appeals and remand for a new trial.
. TexR.Civ.P. 166b(6)(b) provides in full:
If the party expects to call an expert witness when the identity or the subject matter of such expert witness’ testimony has not been previously disclosed in response to an appropriate inquiry directly addressed to these matters, such response must be supplemented to in-elude the name, address and telephone number of the expert witness and the substance of the testimony concerning which the expert witness is expected to testify, as soon as is practical, but in no event less than thirty (30) days prior to the beginning of trial except on leave of court.
. Regarding Barnard's speed, the testimony of the accident witnesses varied — one had no opinion, another would only say "not slow,” and the third estimated "approximately forty miles an hour.”