In a study of 157 men and 141 women with gonorrhoea post-gonococcal urethritis (PGU) in men was significantly more common among chlamydia-positive (76%) than among chlamydia-negative (22.5%) patients. Clinical investigations of PGU detected 95% of the patients infected with C trachomatis. PGU was, however, asymptomatic in over half of the patients and a careful follow-up for 3-6 weeks was necessary to detect between 80% and over 90% of cases. PGU was not related to age, past history of gonorrhoea or non-gonococcal urethritis, severity of gonococcal infection, or chlamydial inclusion count. Although post-gonococcal cervicitis (PGC) in women was an identifiable entity, it was detected in only one-third of chlamydia-positive patients. PGC was significantly associated with the 20-29 year-old age group but was not related to symptoms or chlamydial inclusion count. In the absence of facilities for culturing chlamydia, selection on an epidemiological basis of all female consorts of men with PGU, together with the remaining women with PGC, would have resulted in some unnecessary treatments and left untreated up to 30% of those harbouring C trachomatis.