Cytogenetic analyses in acute myeloid leukemia (AML) and acute lymphoblastic leukemia (ALL) have revealed a great number of non-random chromosome abnormalities. In many instances, molecular studies of these abnormalities identified specific genes implicated in the process of leukemogenesis. The more common chromosome aberrations have been associated with specific laboratory and clinical characteristics, and are now being used as diagnostic and prognostic markers guiding the clinician in selecting the most effective therapies. Specific chromosome aberrations and their molecular counterparts have been included in the World Health Organization classification of hematologic malignancies, and together with morphology, immunophenotype and clinical features are used to define distinct disease entities. However, the prognostic importance of less frequent recurrent aberrations in AML and ALL, both primary and secondary, is still to be determined. This review summarizes current views on clinical relevance of major cytogenetic findings in adult AML and ALL.