An analysis using case reports, laboratory records of tests for C. trachomatis, and Hospital Discharge Summary data shows that, following implementation of a chlamydia prevention program in Wisconsin in 1985, statewide declines were observed in prevalence, incidence and complications of infection. In 1990, prevalence rates among teenage women peaked at 2,794 infections per 100,000 15-19-year-old females. Between 1987 and 1991 (a period of stable testing volume), the proportion of positive tests decreased in all age-groups for females (by 29-41%) and males (by 10-14%), and the incidence of new infections in women decreased in clinic populations by 27%-50%. Between 1986 and 1991, hospitalization rates declined by 33% for pelvic inflammatory disease and by 20% for ectopic pregnancy.