Understanding of the roles and molecular mechanisms of hematopoietic growth factors has increased greatly in recent years. This past decade has also brought us tantalizingly close to linking a group of genes normally involved in the regulation of growth and differentiation--the cellular proto-oncogenes--to the process of malignant transformation. In this article, we review the known actions of hematopoietic growth factors in normal hematopoiesis and in hematologic malignancies. We then discuss current concepts of the roles of proto-oncogenes in normal cells and their potential involvement in leukemogenic events. A merging of these concepts with classical hypotheses of multistage carcinogenesis is emphasized.