Prostaglandins E2 and F2 alpha were measured in decidual tissue of women after spontaneous abortions at pregnancy terms 8 to 10 weeks and after abortions induced by sulprostone, a synthetic prostaglandin E2 analog, at pregnancy term 4-5 weeks. Women in whom pregnancy was terminated surgically at relevant periods were controls. Prostaglandin E2 concentration was found to be 5 times higher and that of prostaglandin F2 alpha 40 times higher in women with spontaneous abortions than in surgical abortion. In sulprostone-induced abortion prostaglandin concentrations in decidual tissue were the same as in controls. These data indicate that a drastic increase of prostaglandin levels in spontaneous abortion is not only a result of myometrial contractions or decidual tissue rejection, but an important component in the chain of events leading to miscarriage.