The clonal malignancies of acute myeloid leukemia and the myelodysplastic syndromes are associated with numerous chromosomal and oncogenic abnormalities. Activation of oncogenes has been demonstrated, although there is little evidence that this alone causes malignant transformation of diploid cells as a consequence. Patterns of abnormalities can be seen as the patient progresses from myelodysplastic syndrome to acute myeloid leukemia, but no unique or invariant findings have been described. Chromosomal changes, with the exception of some translocations, are neither disease nor lineage specific. At this time the data provide good support for the multistep view of carcinogenesis, and there is indirect or circumstantial evidence for the presence of tumor suppressor genes on 5q and 7q. The continued study of these clonal hematological disorders will provide considerable insight into mechanisms of tumorigenesis and possibly may lead to new modes of therapy, for example, through altering the microenvironment, interfering with deranged signal transmission, or introducing antioncogenes.