We have observed isolated hepatotoxicity in two children treated with propylthiouracil (PTU) for hyperthyroidism. Neither patient had risk factors for or clinical evidence of preexisting liver disease. In one patient the drug was promptly discontinued when signs of liver disease were noted. This patient quickly recovered. The second patient continued to receive PTU for several days after developing symptoms. Her illness progressed to fulminant hepatic failure with encephalopathy, and she died. These are the third and fourth pediatric cases reported, and there have been 10 cases reported in adults in the English language literature. Thirteen of the 14 patients are female. The literature regarding all these patients is reviewed. Propylthiouracil may cause lethal hepatic damage. This drug should be discontinued immediately if signs or symptoms of hepatic injury are detected.