The use of immunosuppressive therapy has markedly increased over the past several years, and concomitant with its use has been an increased frequency of associated neoplasia. The patient presented is a 22-year-old white male who, following two renal transplants and prolonged immunosuppressive therapy with azathioprine and methylprednisolone, developed chronic granulocytic leukemia. Chromosome karyotyping demonstrated the somewhat unusual development of a Philadelphia chromosome with translocation to the No. 7 of the C group. A review of transplantation centers revealed that five cases of chronic granulocytic leukemia have occurred in a population of 25,000 renal transplant patients, a 5-fold increased incidence over the general population. Possible etiologies that may be responsible for the development of chronic granulocytic leukemia in patients on immunosuppressive therapy are discussed. It is our hope that by the introduction of these reports of chronic granulocytic leukemia into the medical literature, the need for caution in the use of immunosuppressive drugs in nonmalignant disease will again be emphasized.