A prospective study of 356 consecutive heterosexual male patients attending the Department of Genitourinary Medicine at University College Hospital was carried out to determine the prevalence of Chlamydia trachomatis. Patients were asked about their symptoms, use of condoms and change of sexual partner. The prevalence of non-gonococcal urethritis (NGU--chlamydia positive and negative urethritis) was 37% (131 of 356). C. trachomatis was shown to be the causative organism in 24% (31 of 131) of patients with NGU. The prevalence of other STDs in men with C. trachomatis and with non-chlamydial urethritis was 15% and 10% respectively. Men with C. trachomatis were significantly more likely than men with non-chlamydial urethritis to be asymptomatic (56% vs 35%). The prevalence of C. trachomatis was highest in men who had changed partner in the previous 3 months (20 of 32 men). A third of men never used condoms in the first 3 months of a new relationship and over half failed to use them after 3 months. There was no evidence that the reported use of condoms reduced the rate of infection with C. trachomatis.