Before treatment for urethral gonorrhea, Chlamydia trachomatis was isolated from 18% and Ureaplasma urealyticum from 37% of 121 men. C. trachomatis was recovered from none of 18 homosexual men who had gonorrhea and from 22 of 95 heterosexual men who had gonorrhea (P < 0.05). After treatment with a penicillin, postgonococcal urethritis occurred significantly more often in heterosexual than in homosexual men (P < 0.002). Postgonococcal urethritis developed in all men from whom C. trachomatis was isolated. Among men without U. urealyticum infection, postgonococcal urethritis was significantly associated with C. trachomatis infection (P < 0.02). Among men without C. trachomatis infection, postgonococcal urethritis was less closely associated with U. urealyticum infection (0.1 > P > 0.05). Postgonococcal urethritis was least frequent among men who had neither C. trachomatis nor U. urealyticum infection.