Cellular or proto-oncogenes are normal cellular genes important in normal cell growth and development. In some instances abnormal expression of these genes is associated with altered cell growth or with malignant transformation. Abnormalities of cellular oncogenes are common in human leukemias. These arise by multiple mechanisms such as mutation, translocation, amplification, and others. Sometimes more than one abnormality is present within a single oncogene. In other instances, a leukemia cell may contain abnormalities of several different oncogenes. Some oncogene abnormalities are relatively specific for certain leukemias and occur in almost all cases; examples include ABL in chronic myelogenous leukemia or MYC in Burkitt leukemia/lymphoma. Other abnormalities are also relatively specific but occur in only some cases such as NRAS in acute myelogenous leukemia or BCL2 in B-cell acute lymphoblastic leukemia. In other leukemias, such as most cases of acute lymphoblastic leukemia and chronic lymphocytic leukemia, oncogene abnormalities are uncommon. The precise role of oncogenes in the pathogenesis of human leukemia is unknown. Retrovirus transduced versions of some of the oncogenes modified in human leukemias cause leukemia in animals. Other oncogenes, modified or unmodified, transform animal and human hematopoietic cells in vitro. Some oncogene products are hematopoietic growth factors or growth factor receptors while others regulate cell proliferation or differentiation by diverse mechanisms. Disruption of the balance between these processes seems the most likely mechanism of oncogene related leukemogenesis. If the role of oncogenes in human leukemias can be defined, innovative diagnostic and therapeutic strategies may be forthcoming.