A 59-year-old patient is described who presented with acute lymphocytic leukemia. Complete remission was induced rapidly with vincristine and prednisone. Nine months later he acquired the clinical and hematological picture of a myeloproliferative disease, and bone marrow cytogenetic studies demonstrated the presence of the Philadelphia chromosome. Within 4 months he again developed overt acute lymphocytic leukemia and died. Several lines of evidence have recently indicated a relationship between myeloproliferative and lymphoproliferative disorders, particularly chronic myelocytic and acute lymphocytic leukemia. This patient's course is another example of that association.