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.