To evaluate the association between ectopic pregnancy and 22 potential risk factors, we conducted a population-based case-control study. The investigation included 274 cases diagnosed from 1935 through 1982 in residents of Rochester, Minn, and 548 matched controls selected from live-birth deliveries. Risk factor information documented prior to the last index menstrual period was obtained via medical record abstract. Univariate matched analyses revealed nine variables associated with a significantly elevated relative risk of ectopic pregnancy. Following conditional logistic regression, four variables remained as strong and independent risk factors for ectopic pregnancy: current intrauterine device use (relative risk, 13.7; 95% confidence interval, 1.6 to 120.6), a history of infertility (relative risk, 2.6; 95% confidence interval, 1.6 to 4.2), a history of pelvic inflammatory disease (relative risk, 3.3; 95% confidence interval, 1.6 to 6.6), and prior tubal surgery (relative risk, 4.5; 95% confidence interval, 1.5 to 13.9). Theoretically, any condition that prevents or retards migration of the fertilized ovum to the uterus could predispose a woman to ectopic gestation. Further research is needed to clarify the impact of other potential risk factors in the etiology of ectopic pregnancy.