T-mycoplasmas have been associated with nongonococcal urethritis (NGU) in studies in which these organisms were found to be more prevalent among men with NGU than among control groups of men. In none of these studies were the groups matched for sexual experience, a variable which we have shown to be an important determinant of colonization with T-mycoplasmas. We obtained urethral cultures for genital mycoplasmas from men presenting to the Boston City Hospital with gonococcal urethritis and with NGU, and from men of comparable sexual experience who did not have urethritis. Colonization with T-mycoplasmas was no more prevalent among men who had NGU than among the men who did not have urethritis. These data raise some serious questions about the role of T-mycoplasmas in nongonococcal urethritis.