In 1982, the French-American-British (FAB) cooperative group proposed a classification of myelodysplastic syndromes (MDS) based on morphological features in blood and bone marrow, namely on medullary and peripheral blast count, Auer rods, ring sideroblasts and the number of monocytes in the peripheral blood. This classification has been used for numerous studies regarding morphology, prognosis and treatment of MDS. Some details of this morphological classification remained unclear, and some patients were unclassifiable. A working group of the World Health Organization (WHO) recently proposed a new classification of MDS, based on a significant modification of the original FAB proposals. CMML and RAEB-T were removed from the MDS classification and RAEB was split into two groups with medullary blast counts below and above 10%. In addition, a group of patients with less than 5% medullary blasts but evidence of multilineage dysplasia was defined. MDS patients with 5q- as the sole chromosomal anomaly were also considered a separate group. The aim of the present study was to validate the new classification with respect to prognostic importance, and to correlate it with cytogenetic and hematological features in a large series of patients (n=1600) with a long-term follow up. We were able to confirm a significant difference in prognosis between RAEB I and RAEB II, as well as a difference between refractory anemia and multilineage dysplasia. Furthermore, patients with 5q- anomaly had a much better prognosis than other WHO subtypes, but this was only true for patients with a medullary blast count below 5%. In summary, the WHO classification appears to define morphological subgroups that are more homogeneous with respect to prognosis than the FAB subtypes.