In order to study the effects of metronidazole on the microbiology of the vagina in general and on Gardnerella vaginalis infection in particular, quantitative aerobic and anaerobic bacterial cultures were performed before and 4 weeks after initiation of metronidazole treatment, 400 mg three times daily for five days. Bacteriological results were compared with microscopic findings and with the results of "amine testing" with potassium hydroxide and with other clinical variables. We found a reasonably good correlation between the finding of clue cells, positive amine test and symptomatic Gardnerella infection. Treatment with metronidazole resulted in most cases in recolonization with lactobacilli and disappearance of clinical symptoms and findings, including clue cells, although G. vaginalis could still be detected.