Prostaglandins appear to mediate the uterine contractions of abortion and labor, and aspirin has been shown to be an inhibitor of prostaglandin synthesis. In this double-blind, placebo-controlled, prospective, and randomized study, aspirin was administered orally in doses of 600 mg. every 6 hours to patients undergoing induced midtrimester abortions with hyperosmolar urea and oxytocin augmentation. The mean injection-abortion interval was significantly prolonged by aspirin in nulliparous patients (aspirin 32.3 +/- 3.3 hr. vs. placebo 21.5 +/- 3.5 hr.) and no aspirin-treated nullipara aborted in less than 18 hours. There was no significant difference between treatment groups in the mean injection-abortion interval in the multiparous patients at the dose of aspirin used. The effectiveness of aspirin in the prolongation of the injection-abortion interval has potential therapeutic implications for the treatment of premature labor.