Fifty women with ectopic pregnancy and 49 control women with intrauterine pregnancy were interviewed and evaluated for evidence of Chlamydia trachomatis infection. Among women with ectopic pregnancy, 14 women were wearing an intrauterine contraceptive device or had a tubal ligation (group A), and 36 women had no readily identifiable risk factors (group B). Group B women had greater total numbers of sexual partners than did control women with intrauterine pregnancy (P less than .005). Group B women more often had C trachomatis antibody than group A (P = .03) and control women (P = .002). Of 27 C trachomatis cultures from fallopian tube tissue from women with ectopic pregnancy, none were positive. Fallopian tube tissue distant from the site of ectopic implantation was available for histopathology of 41 cases. Nine (22%) had extensive subepithelial plasma cell infiltration. All nine were among group B women (P = .06) and all seven with plasma cell salpingitis who were tested for C trachomatis antibody were seropositive (P = .004). It is concluded that a subset of women with ectopic pregnancy were at increased risk for acquiring a sexually transmitted disease by virtue of their sexual behavior and that women in this subset frequently have serologic evidence of C trachomatis infection and histologic evidence of plasma cell salpingitis. Because few of these women recall having had pelvic infection, the authors speculate that subclinical C trachomatis tubal infection producing plasma cell salpingitis may commonly underly ectopic pregnancy.