We conducted a multi-center clinical study to evaluate the role of Ureaplasma urealyticum and Mycoplasma hominis in male urethritis. The incidence of each organism in first-voided urine samples of 160 male urethritis patients, including 28 with gonococcal and 126 with non-gonococcal urethritis, was investigated. U. urealyticum and M. hominis were isolated from 13.6% and 6.5%, respectively, of the urine samples, and the concentration of each mycoplasma exceeded 10(3) ccu/ml (color changing units/ml) in 5.2% and 3.9%, respectively. Among 64 patients with non-gonococcal non-chlamydial urethritis, U. urealyticum and M. hominis concentration exceeding more than 10(3) ccu/ml were detected in only two and one samples, respectively. The incidence among urethritis patients with a concentration of either mycoplasma exceeding 10(3) ccu/ml was not significantly greater than that among subfertile males without urethritis. The findings of the present study suggest that, although mycoplasmas may cause urethritis in some patients, the incidence of urethritis due to U. urealyticum or M. hominis is low among patients with non-gonococcal, non-chlamydial urethritis.