A man received a cadaver renal allograft for end-stage renal failure. After 35 months of immunosuppressive therapy with azathioprine and prednisone, he developed septicemia and a high leukocyte count. In spite of successful treatment of the infection, the leukocyte count continued to rise and a diagnosis of Philadelphia chromosome positive chronic granulocytic leukemia was made. An increased incidence of malignant disease, especially lymphoreticular malignancy, is well described in immunosuppressed patients with allografts. However, the association of chronic granulocytic leukemia and immunosuppressive therapy previously has not been reported. An additional etiological factor in this patient may have been the extensive diagnostic radiological investigations undertaken in childhood. The recent addition of allopurinol to the immunosuppressive therapy has normalized the platelet and leukocyte counts, probably by potentiating mercaptopurine.