Order, Supreme Court, New York County (Stanley Sklar, J.), entered March 23, 1995, which granted defendant New *128York City Health and Hospitals Corporation’s (HHC) motion to dismiss the complaint pursuant to General Municipal Law § 50-e, unanimously affirmed, without costs.
The IAS Court properly dismissed the complaint in this medical malpractice action on the ground of plaintiffs’ failure to properly serve a notice of claim upon HHC within 90 days of the accrual of the cause of action, as required by the notice of claim provisions of General Municipal Law § 50-e and McKinney’s Unconsolidated Laws of NY § 7401 (2) (Health and Hospitals Corporation Act § 20 [2] [L 1969, ch 1016, § 1, as amended]; see, Nicholas v City of New York, 130 AD2d 470). Service by plaintiffs of the notice of claim upon the City of New York, rather than upon HHC, does not constitute compliance with the notice of claim requirements for service upon HHC (Brennan v City of New York, 59 NY2d 791, 792; Kroin v City of New York, 210 AD2d 95, 96).
The IAS Court also properly determined that HHC was not estopped from asserting plaintiffs’ non-compliance, inasmuch as the doctrine of equitable estoppel is to be invoked sparingly and only under exceptional circumstances (Matter of Gross v New York City Health & Hosps. Corp., 122 AD2d 793, 794), and since HHC was under no duty to raise plaintiffs’ failure to properly serve a notice of claim on HHC as an affirmative defense or otherwise bring to plaintiffs’ attention that the City of New York was not a proper party defendant (Kroin v City of New York, supra). While a municipal corporation may, by its conduct, waive an irregularity in the notice of claim, the requirements as to the manner or time of service may not be waived (supra).
We have considered plaintiffs’ remaining arguments and find them to be without merit. Concur — Rosenberger, J. P., Wallach, Rubin, Kupferman and Mazzarelli, JJ.