Severe acute and chronic hepatic damage occurred in a white man who had taken phenelzine sulfate (Nardil) 45 mg daily for 70 days. Liver biopsy showed a mixed hepatitic and cholestatic pattern with extracellular deposition of a unique homogeneous collagenous substance. Portal cirrhosis developed and has persisted. The patient was found to have a "rapid acetylator phenotype" and high rate of metabolism of antipyrine. These innate factors may have predisposed to hepatic injury due to phenelzine.