Postgonococcal urethritis (PGU) occurred in 69 (21.3%) of 344 men with urethral gonorrhoea. Each case of gonorrhoea was treated with intramuscular procaine penicillin 2.4 megaunits and the penicillin sensitivity was determined. All 69 of the isolates from patients with PGU were fully sensitive to penicillin (IC50 less than 0.08 iu penicillin per ml). Twenty-eight (10.2%) of 275 cases of gonorrhoea which were not followed by PGU were relatively resistant to penicillin. The persistence of gonococcal L-forms is therefore not likely to be a main cause of PGU. An incidental finding was that significantly more of the 69 men who developed PGU had a history of non-specific urethritis compared with the 224 men who fully recovered (P less than 0.01).