Gonorrhea (GU) and nongonococcal urethritis (NGU), the commonest sexually transmitted diseases in men, are endemic at high levels in most regions of the world, despite effective methods of diagnosis and treatment. NGU is a milder disease with a longer incubation period and longer duration of symptoms before treatment. Although Chlamydia trachomatis and Ureaplasma urealyticum appear to account for a majority of NGU cases, the cause of approximately 20% of cases remains unknown. NGU is about twice as common as GU in Western countries, and on the basis of national statistics from Great Britain, its incidence has been thought to be increasing more rapidly. However, data from individual clinics suggest that incidence of both diseases has remained stable over the past decade. The increased incidence of NGU in Great Britain over the past decade probably results from increased recognition or reporting. Marked socioeconomic and racial differences in both relative incidence of and susceptibility to NGU and GU have been found repeatedly but are unexplained. Men appear to transmit GU to women more efficiently than vice versa. Neisseria gonorrhoeae appears to be more infectious than C. trachomatis for both men and women, but interpretation of differences in susceptibility and transmission is hindered by ignorance of natural and acquired immunity to these agents. Of the several important time intervals in chains of transmission of venereal urethritis, only incubation periods and iatrotropic intervals for GU and NGU and the posttreatment period of infectivity for GU are known.