The recent WHO classification for acute leukemias and Myelodysplastic syndromeseparates entities by recurrent cytogenetic abnormalities and immunophenotypic featurespresenting prognostic impact. These are diagnosed by peripheral blood counts, bonemarrow cytology, cytochemistry, cytogenetics and immunophenotyping. A number ofrisk factors has been identified that characterize patients at diagnosis as well as duringtheir clinical course. At diagnosis these parameters include age, cytogeneticconfiguration, hemopoietic elements, and a previous history of myelodysplasia orcytoreductive therapy for a pre-existing malignancy. They are complemented by theresponse to induction therapy and subsequent treatments as well as by the duration ofresponseRoutine cytogenetic analysis provides important information on diagnostic andprognostic relevance for hematological malignancies. However, it is often difficult toobtain good karyotypes, especially of cells from cases with acute leukemia (e.g. ALL) ofpoor morphology and spreading. On the other hand, Complex chromosomal aberrationsare present in < or =30% of patients with primary myelodysplastic syndrome (MDS) oracute myeloid leukemia (AML) and are associated with a poor prognosis. Specificalterations in complex karyotypes are defined by novel cytogenetic techniques ascomparative genomic hybridization (CGH), spectral karyotyping (SKY) and fluorescencein situ hybridization (FISH) with selected probes.