Chlamydia trachomatis was isolated from the uretha of 125 (52%) of 238 men with non-gonococcal urethritis (NGU). Repeat isolation attempts in 155 of these patients were successful in eight men in whom results had been negative on the initial visit, but they were unsuccessful in eight men who initially had had positive cultures. We must assume that with our present isolation techniques we are missing, at any single visit, at least 9% of chlamydial infections. C. trachomatis was also found in 32 (23%) of 139 men with gonorrhoea. Positive cultures were obtained from 15 (79%) of 19 men, who later developed post-gonococcal urethritis (PGU). Thiamphenicol, used for the treatment of gonorrhoea, was shown to have very little effect on C. trachomatis, which could still be recovered after treatment in 76% of the patients who initially had had a combined infection. The typing of 35 genital isolates by micro-immunofluorescence confirms the previously reported distribution of chlamydial serotypes. In this study a social profile is given of our patients with urethritis and a comparison is made of the duration of symptoms and the nature of discharge in men with gonococcal, chlamydial, and non-specific urethritis. We were able to show a clear difference in clinical symptoms in men with gonorrhoea and NGU, taken as a whole, but found only a slight difference between men with chlamydial and non-specific urethritis.