Although up to about half the cases of acute non-gonococcal urethritis (NGU) are caused by Chlamydia trachomatis organisms (chlamydiae) and a smaller, ill-defined, proportion probably by Ureaplasma urealyticum organisms (ureaplasmas), the aetiology of all cases is not understood. Clarification of the role of the anaerobe, Bacteroides ureolyticus, was sought in the current study. Seventy five chlamydia negative patients with NGU were treated on a double blind placebo controlled basis with metronidazole. After seven days more of the 35 patients given this drug tended to improve clinically than the 40 given the placebo, but the difference was not significant. Of 23 chlamydia negative but anaerobe positive men, however, 78% (7/9) receiving metronidazole responded clinically, but only 7% (1/14) receiving placebo responded (p less than 0.001). Furthermore, whereas 78% of the anaerobe positive men given metronidazole recovered, only 23% (6/26) of the anaerobe negative men did so (p less than 0.02). No further evidence for the role of ureaplasmas in the aetiology of NGU was obtained, but the data suggest that B ureolyticus organisms, and perhaps other anaerobes, have an important role in a small proportion of cases and that the beneficial effects of metronidazole given on an empirical basis will be confined to anaerobe positive urethritis.