A malignant hepatoma occurred in a 12-year-old girl who eight years previously had developed an acute lymphoblastic leukaemia which for eight years had been in complete haematological remission. Fourteen months after the last re-induction treatment period had been discontinued, but while on methotrexate and 6-mercaptopurine maintenance, a hepatocellular liver carcinoma developed of which the patient died after a fulminating course, still in complete haematological remission. As far as is known, no direct carcinogenic effect can be ascribed to the two antimetabolites, but it must be assumed that these two drugs, taken by the patient for over seven years, led to cirrhosis of the liver whose malignant transformation was significantly influenced by the immunosuppressive effects of methotrexate and 6-mercaptopurine, given as maintenance therapy according to protocol 02 LA 64, Paris.