Because of the reduced risk of ovarian cancer related to prior full-term pregnancies, we sought to determine whether there was any association with a history of one or more incomplete pregnancies. White female residents of three counties in Washington State (United States) diagnosed with ovarian cancer during 1986-88 (n = 322), and a random sample of control women selected from these same counties (n = 426), were interviewed regarding their pregnancy and childbearing histories. Among women who had given birth to at least one child, an additional incomplete pregnancy was not associated with the risk of ovarian cancer (relative risk [RR] = 1.1, 95 percent confidence interval [CI] = 0.8-1.6, adjusting for age, oral contraceptive use, and number of births). For those who had never given birth, a somewhat smaller proportion of cases had a history of incomplete pregnancy than controls (RR = 0.8, CI = 0.4-1.7). In an analysis restricted to ever-pregnant women, a prior induced or spontaneous abortion was not found to be associated with the incidence of ovarian tumors (RR = 1.0, CI = 0.6-1.7, and RR = 1.3, CI = 0.8-1.9, respectively). Other studies of the possible relation between incomplete pregnancies and ovarian cancer generally have observed either a weak negative association or no association at all. It is possible that if incomplete pregnancies do affect the risk of ovarian cancer, their impact might be too small to be identified reliably through epidemiologic studies.