Metronidazole is a drug used for the treatment of trichomonal vaginitis, amebiasis, giardiasis, and certain anaerobic bacterial infections in humans. Acetamide and N-(2-hydroxyethyl)oxamic acid are metabolites of metronidazole in the rat, and we find small amounts of both metabolites in the urine of human patients taking the drug. Although acetamide is carcinogenic for rats, we do not believe that our finding further defines metronidazole's risk for humans. That risk can only be estimated from surveillance of people previously exposed to the drug.