Chlamydia trachomatis was isolated from 26% of urethral swabs taken from 509 men with urethritis. The highest yield of 68% was obtained from a selected group of men with nonspecific urethritis (NSU) who had a frank urethral discharge. This is a higher than in previous reports, and is significantly higher than the isolation of C. trachomatis from men with less severe urethritis. The higher yield was similar to C. trachomatis isolation rates reported among patients with severe trachoma in hyperendemic areas. Men with a previous history of NSU had low isolation rates. Overall, 30% of 385 men with NSU had positive chlamydial culture results, 7% of 59 men with gonococcal urethritis alone were Chlamydia-positive, 15% of 59 men with gonorrhoea followed by NSU (post-gonococcal urethritis) were Chlamydia-positive, and only 3% of 61 men without urethritis harboured Chlamydia. Swabs taken from the cervical os of 28 of 108 female contacts of men with NSU had a positive result for C. trachomatis. Significantly more pairs of sexual partners had the same chlamydial culture result than had different results. The chlamydial isolation rate was higher among men admitting a casual sexual contact than in men claiming only regular partnerships. The findings provide further evidence for the sexual transmission of C. trachomatis and for its aetiological role in NSU.