Four patients are presented who developed toxic hepatitis while being treated for leukemia with 6-mercaptopurine and steroids. The onset was rapid and suggested an acute hepatitis with intrahepatic blockage. Pathologic examination of the liver in two patients who expired with hepatitis is presented and the pathology described. In the third autopsied case the hepatitis had resolved while receiving massive steroid therapy, even though the leukemia progressed rapidly when 6-mercaptopurine was withdrawn. Early recognition of the toxic hepatitis becomes especially important whero 6-mercaptopurine is the first drug to be used in the management of the patient with acute leukemia.