Hematologic malignancy has rarely been reported in adults with sickle cell disease. We describe four sickle cell patients (two with hemoglobin SC, two with hemoglobin SS) who developed hematologic malignancy (acute myeloblastic leukemia, multiple myeloma, malignant histiocytosis, and Hodgkin's disease). Three of the cases represent the first adult association between SC or SS hemoglobinopathy and the particular malignancy involved. Sickle hemoglobin does not appear to exert a protective effect against childhood hematologic malignancies, suggesting that better survival in sickle cell disease may be accompanied by an increased incidence of hematologic neoplasms in adulthood. Karyotypic analysis revealed alterations of chromosome 5 in two sickle cell patients with leukemia, raising the possibility of a chromosomal link between the two diseases. Further epidemiologic and cytogenetic studies are needed to define the relationship between hematologic malignancy and sickle cell disease.