Cytogenetic analysis is the most important diagnostic tool for determining prognosis in acute myeloid leukemia (AML). In the majority of patients with AML, acquired clonal chromosome aberrations can be observed. Numerous recurrent karyotype abnormalities have been discovered in AML. These findings on the chromosomal level have paved the way for molecular studies that have identified genes involved in the process of leukemogenesis. The identification of specific chromosomal abnormalities and their correlation with cytomorphologic features, immunophenotype, and clinical outcome have led to a new understanding of AML as a heterogeneous group of distinct biologic entities. The importance of cytogenetic findings in AML for classification and for the understanding of pathogenetic mechanisms is increasingly appreciated in the clinical context, and the new World Health Organization classification of AML uses cytogenetic abnormalities as a major criterion.