This case-control study was carried out to evaluate the significance of genital infections as risk factors in ectopic pregnancy. The study population consisted of 86 consecutive patients with ectopic pregnancy and two age- and parity-matched control groups of women with normal early pregnancy; those with planned pregnancy (I; 65 pairs) and those having legal abortion (II; 51 pairs). Histories of salpingo-oophoritis in the two groups (22% against 5%, P less than 0.05), and of cervical Chlamydia trachomatis infection (9% against 0%, P less than 0.05) were more common in patients with ectopic pregnancy than in their controls with planned pregnancy; women in the control group who had undergone legal abortion did not differ from patients with ectopic pregnancy. In serological studies antibodies against Herpes simplex viruses were commoner in patients with ectopic pregnancy than in both control groups (89% against 62%, and 88% against 57%, P less than 0.001). Antibodies against cytomegalovirus were also commoner in patients with ectopic pregnancy than in controls who had had a planned pregnancy (88% against 72%, P less than 0.05). The results support the concept that gynaecological infections are among factors predisposing to ectopic pregnancies.