Urethral specimens from 726 patients with nongonococcal urethritis (NGU) were examined for Chlamydia trachomatis, Ureaplasma urealyticum, and Mycoplasma hominis. Chlamydiae were isolated from 35.9% of ureaplasma-positive patients and from 36.5% of ureaplasma-negative patients. Ureaplasmas were isolated from 52.5% of chlamydia-positive patients and from 53.1% of chlamydia-negative patients, an observation which contrasts with that of some workers who have suggested that ureaplasmas are significantly associated with chlamydia-negative NGU. Furthermore, the numbers of ureaplasmas isolated from patients who did or did not harbour chlamydiae were not significantly different nor was there a particular association of ureaplasmas with chlamydia-negative NGU in patients experiencing their first episode of disease. In addition, M. hominis was not isolated more frequently from those from whom chlamydiae were or were not isolated. The only significant associations were the isolation of M. hominis from patients who were ureaplasma-positive and of ureaplasmas from those who were M. hominis-positive. These findings do not necessarily mitigate against ureaplasmas being responsible for some cases of chlamydia-negative NGU.