Four cases of acute myelogenous leukemia and six cases of chronic myelogenous leukemia after treatment with azathioprine and prednisone for renal allotransplantation have been described in the literature. We report another two cases of acute leukemia 10 and 5 years after successful renal allotransplantation. Patient 1, a 29-year-old farmer, exhibited the signs of acute lymphatic leukemia resistent to treatment with cytostatic agens. Death was due to pneumonia. Patient 2, a 47-year-old salesman, developed pancytopenia together with splenomegaly. After splenectomy an atypical subacute myeloid leukemia became apparent which was not treated due to withdrawal of the patient. He died 2 months after diagnosis. Both patients received long-term immunosuppressive therapy with azathiopine and prednisone until the leukemia was diagnosed. A relationship between long-term immunosuppression and the occurrence of leukemia is postulated.