Chlamydia trachomatis can be identified in up to 60% of cases of nongonococcal urethritis (NGU) and the aetiology of most of the remainder is obscure. This paper reports a role for other genitourinary (GU) infections such as candidiasis, warts and herpes simplex, in the causation of NGU. One hundred and ten men fulfilled the entry criteria which included the probability that their contacts would attend the department. Fifty-four of the 110 men had GU infections other than NGU; 56 had no other infections. NGU was detected in 31 (57%) of cases with other GU infections and 8 (26%) were chlamydiae-positive: in contrast NGU was found in only 10 (18%) of those with none of the other infections and 3 (30%) were chlamydiae-positive. Chlamydiae-negative NGU was, therefore, more common in those with other GU infections. Forty-five (86%) of 52 contacts of 41 patients with NGU had various GU infections such as candidosis and anaerobic vaginosis, in contrast with other GU infections in only 7 (23%) of 30 contacts of men with no other GU infections. Other GU infections in patients and their contacts appeared significantly related to the presence of chlamydiae-negative NGU.