The oxidation of sulfamethoxazole to its hydroxylamine metabolite was investigated in vitro with human liver microsomes and in vivo by detection in the urine. Sulfamethoxazole was oxidized to the hydroxylamine in an NADPH-dependent process by liver microsomes prepared from two human livers. Three healthy volunteers ingested 1000 mg sulfamethoxazole, and urine was collected for 24 hours. Sulfamethoxazole hydroxylamine constituted 3.1% +/- 0.7% of the drug excreted in the urine in 24 hours. Fifty-four percent of the ingested dose was excreted during this same time period. We conclude that sulfamethoxazole hydroxylamine is an authentic in vivo metabolite in humans, probably formed predominantly by cytochrome P450 in the liver. It could be responsible for mediation of sulfonamide adverse reactions, particularly hypersensitivity reactions.