This review focuses on karyotypic and molecular findings of myelodysplastic syndromes (MDS). Genetic entities are distinct on the basis of structural (deletions, translocations, inversions) or numerical chromosomal abnormalities (trisomies, monosomies). New information about the amount and nature of malignant cells in MDS, as well as of genes rearranging in specific translocations, recently provided by molecular cytogenetics, are analysed. Integration of clinical-haematological classifications with cytogenetic and molecular findings is discussed