In a case-control study, chlamydial antibody (IgG) was examined in sera from 58 women with ectopic pregnancy and 58 birth-giving controls matched for age (+/- 3 years) and parity. More patients than controls stated that they had previously been treated for salpingitis (p less than 0.05). In the index pregnancy, two cases and four controls were positive for C trachomatis in specimen from the cervix uteri. Chlamydial antibody was detected among 69% of the cases and 33% of the controls (p less than 0.01). These results were consistent with age, parity and information on previous treatment for salpingitis. The occurrence of a previous chlamydial infection, proven by IgG-antibodies, increases risk of an ectopic pregnancy by a factor of 4.6 (odds ratio 4.6, 95% CI 2.1-10.3).