Cytogenetic analysis of leukemic cells has been shown to be a mandatory part of the diagnosis of malignant hemopathies. Recurring abnormalities may be divided into those exclusively found in myeloid disorders, those associated with lymphoid diseases and those detected in both types of hemopathy. Several of the common defects are characteristic of specific FAB types or subtypes and associated with clinico-pathologic manifestations. Cytogenetic abnormalities have served to identify relatively homogenous subsets of malignant hemopathies. In view of the significant progress realized in the treatment of malignant hemopathies, the assessment of prognostic factors is particularly important for therapeutic decision making. The chromosome status at diagnosis has proven to be a major prognostic indicator for survival and outcome in individual patients. This article will focus on the diagnostic and prognostic significance of the karyotype in de novo acute leukemia and primary myelodysplastic syndromes.