Denial of the motion to amend, made 15 years after the alleged malpractice, 12 years after the initial bill of particulars, and more than four years after its first amendment, was a proper exercise of the court's discretion. Not only did plaintiffs fail to offer a reasonable excuse for this inordinate delay, but the proposed amendment had no merit ( see Katechis v Our Lady of Mercy Med. Ctr, 36 AD3d 514, 516), propounding material changes that prejudicially introduced a new theory of liability ( Vega v Lenox Hill Hosp., 235 AD2d 302) that defendant had failed to diagnose an incompetent cervix, inconsistent with the previously alleged theory of failure to diagnose a bacterial infection.