A case-control study was conducted in 1988 in seven Paris area maternity hospitals to evaluate the role of several risk factors, particularly infectious factors, in ectopic pregnancy. A total of 279 cases and 279 controls were compared for sociodemographic characteristics, cigarette smoking, sexual, reproductive and surgical histories, and conditions of conception. Pelvic inflammatory disease confirmed by celioscopy (odds ratio (OR) = 5.5, 95% confidence interval (CI) 2.1-13.9) and Chlamydia trachomatis seropositivity (OR = 3.9, 95% CI 2.3-6.7) appeared to be important risk factors for ectopic pregnancy. Other risk factors found to be associated with an increased risk of ectopic pregnancy were dose-related cigarette smoking at the time of conception (ORs 1.3 to 2.5), appendectomy (OR = 1.6, 95% CI 1.1-2.5), prior tubal surgery (OR = 5.1, 95% CI 1.7-15.4), induced conception cycle (OR = 3.2, 95% CI 1.1-9.3), and prior ectopic pregnancy (OR = 13.3, 95% CI 4.5-39.2). However, some of the latter risk factors, i.e., prior tubal surgery, prior ectopic pregnancy, and perhaps appendectomy, may be considered to be the results of pelvic inflammatory disease and sexually transmitted diseases. Maternal age, parity, prior induced abortion, and prior spontaneous abortion were not associated with ectopic pregnancy. Use of intrauterine device, progestagen micropill, and also combined estroprogestative pill at the time of conception were associated with a better prevention of intrauterine pregnancy than of ectopic pregnancy. These findings confirm the importance of several previously reported risk factors of ectopic pregnancy: sexually transmitted diseases, cigarette smoking, and prior ectopic pregnancy. They also identified new risk factors, appendectomy and induced conception cycle, and revealed that the combined estroprogestative pill does not prevent ectopic pregnancy as effectively as it does intrauterine pregnancy.