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The myelodysplastic syndromes (MDS) are a group of closely related disorders characterized by chronic cytopenias with cellular marrow, poor prognosis and refractoriness to treatment. We studied 101 consecutive cases of MDS diagnosed over a 7-year period. Peripheral blood (PB) and bone marrow (BM) samples were reviewed and classified according to the proposals of the French-American-British (FAB) cooperative group for MDS. The combined analysis of the initial laboratory features and qualitative haematological abnormalities readily allowed the distinction between the different subgroups. Thirty-two of 79 cases (40.5%) evolved towards other diseases, frequently acute leukaemia (24/79, 30%), or transformed into other MDS (7/79, 9%). In five cases, initially classified as refractory anaemia (RA) or refractory anaemia with ring sideroblasts (RAS), a transitory change to another type of MDS--two chronic myelomonocytic leukaemias (CMML), two refractory anaemias with excess of blasts (RAEB) and one refractory anaemia with excess of blasts 'in transformation' (RAEB-t)--was observed before the evolution towards acute leukaemia. This provides a new link between all these syndromes and increases the number of transitions to other MDS. Overall prognosis was very poor, with differences between subgroups. RA had the best prognosis whereas RAEB-t had the worst one. This study shows that the FAB classification is readily usable and defines well-characterized subgroups of MDS, although there are frequent transitional forms, and as the natural history of the MDS unfolds they may convert into another. The actual poor prognosis and the frequent evolution towards acute leukaemia makes necessary to investigate new methods of treatment for these disorders.

New diagnostic criteria for the diagnosis of the various myelodysplastic syndromes (MDS) are proposed, and a detailed description is given of the features that may help define MDS. Five MDS are described: (1) refractory anaemia (RA), (2) RA with ring sideroblasts, (3) RA with excess of blasts (RAEB), (4) chronic myelomonocytic leukaemia (CMML), and (5) RAEB 'in transformation'. One of the main distinguishing features of these conditions is the proportion of blast cells in the peripheral blood (PB) and/or bone marrow (BM). The morphological features of the blast cells that are of diagnostic importance have been redefined. In RA, with or without ringed sideroblasts, there are fewer than 1% of blasts in the PB and fewer than 5% in the BM; RAEB is defined as having between 5% and 20% of blasts in the BM and fewer than 5% in the PB; RAEB in transformation (a newly defined category) will be considered when any of the following features is present: (i) more than 5% of blasts in the PB, (ii) 20-30% in the BM, and (iii) the presence of Auer rods in granulocyte precursors in BM or PB. In accordance with these newly defined criteria, it is now proposed that over 30% of bone marrow blasts will suffice for the diagnosis of acute myeloid leukaemia (AML) in any of its forms (M1-M6). The proposed descriptions of the MDS should facilitate the interpretation of data emerging from cytogenetic and bone marrow culture studies and the search for features of possible prognostic significance. Recognition of the new category, RAEB in transformation, may throw light on the pathogenesis of AML.