Massive hepatic necrosis following exposure to phenytoin and trimethoprim-sulfamethoxazole is a rare occurrence and to the best of our knowledge has not been reported previously. Acute hepatic failure following administration of trimethoprim-sulfamethoxazole has rarely been seen, and only 4 cases have been well documented pathologically. We report a case of acute liver failure in a 60-year-old woman following ingestion of phenytoin and trimethoprim-sulfamethoxazole concomitantly over a 9-day period. Autopsy findings revealed acute fulminant hepatic failure. This case demonstrates the effects of chemical-chemical interactions in the potentiation of hepatotoxicity of single agents and specifically illustrates the need for discontinuing trimethoprim-sulfamethoxazole in the presence of early liver injury.