We performed a case-control study of the association of past exposure to Chlamydia trachomatis and ectopic pregnancy with 306 case patients with an ectopic pregnancy and 266 pregnant patients who served as controls. The geometric mean antichlamydial antibody titer among cases was 75 +/- 10.2 vs 13 +/- 11.0 among controls. The matched-pair odds ratio for ectopic pregnancy and IgG titer of 1:64 or greater to C trachomatis was 3.0 (95% confidence interval, 2.1 to 4.4). Adjusting for age at first intercourse, total lifetime partners, douching, history of infertility, and parity yielded a relative risk of 2.4 (95% confidence interval, 1.5 to 3.3). Current douching remained an independent risk factor after controlling for chlamydial exposure, with an adjusted relative risk of 2.1 (95% confidence interval, 1.3 to 3.5). The population attributable fraction for chlamydial infection was 0.47 and that for douching was 0.45. The results stress the need for control of C trachomatis infections and for further study of specific douching behaviors as risk factors for ectopic pregnancy.