In a study of the epidemiology of Chlamydia trachomatis infection in Manitoba during 1981-1990, we retrospectively reviewed laboratory and clinical case notification records as well as hospital and health insurance data concerning pelvic inflammatory disease and ectopic pregnancy. After implementation of a control program in 1987, the annual incidence of chlamydial infection was highest among females aged 15-24 years (3,418 cases per 100,000 residents). Recurrent infection, which occurred in 13.4% of patients, was more common in women (P < .001), patients aged 15-24 years (P < .001), registered North American Indians (P < .001), and persons with concomitant gonorrhea (P < .001). Risk factors for dual (chlamydial and gonococcal) infection included male sex (P < .001) and young age (P < .001). Although the incidence of hospitalization and outpatient visits for pelvic inflammatory disease decreased (P < .001) from 1981 to 1990, the annual incidence of ectopic pregnancy increased from 10 to 16 cases per 1,000 reported pregnancies (P < .001). Control activities focusing on the primary prevention of C. trachomatis infection are presented. Strategies for improving secondary prevention (through case detection and treatment of lower genital infection) include the targeting of individuals with recurrent and multiple sexually transmitted diseases.