The experience of screening of 472 UK family planning patients for Chlamydia trachomatis at the time of their scheduled cervical smear suggests that such screening should become routine. In this particular series the incidence of chlamydial infection was 9.1% in women under 30 years of age and 8.0% in the sample as a whole (up to age 35 years). The incidence was highest in the 21-25-year age group and in users of barrier methods of contraception and the pill. 5% of the 379 asymptomatic subjects were positive for chlamydia. Vaginal discharge and genital warts were the symptoms most commonly associated with a positive diagnosis. Of the 3 methods of chlamydia detection assessed-- culture immunofluorescent antigen straining and enzyme-linked immunoassay (ELISA)--the ELISA test was most sensitive (96% of cases identified). A cost-benefit analysis further supports the feasibility of routine chlamydia screening. The costs of not screening include consultations and treatment for undiagnosed pelvic pain and menstrual irregularity hospital admissions for chlamydia cases that progress to pelvic inflammatory disease or lead to ectopic pregnancies and expensive infertility investigations and treatments. The cost- effectiveness can be improved by targeting screening to women at greatest risk--those under 35 years of age.