Fifty-five women with ectopic pregnancy and 24 undergoing tubal ligation with a segmental resection of the fallopian tube were evaluated for histopathology of the fallopian tube, Chlamydia trachomatis serum antibodies, antibodies to a chlamydial sarkosyl-soluble 57-kDa protein, and for isolation of C. trachomatis. Plasma cell infiltration in the fallopian tube submucosa was identified in 31 (65%) of 48 women with ectopic pregnancies and in 8 (33%) of 24 undergoing tubal ligation (P = .01; odds ratio [OR], 3.6; 95% confidence interval [CI], 1.3-10.3). Plasma cell infiltration was correlated with C. trachomatis seropositivity among women with ectopic pregnancy (P = .005; OR, 7.2; 95% CI, 1.7-31) and among women undergoing tubal ligation (P = .008). Of 21 C. trachomatis-seropositive women with ectopic pregnancies, 19 had antibodies to the 57-kDa antigen compared with 1 of 4 seropositive women having tubal ligation (P = .008). Immune responses to the 57-kDa antigen may be involved in the immunopathogenesis of C. trachomatis-associated ectopic pregnancy.