The WHO classification subdivides the FAB RAEB category into RAEB-1 (bone marrow (BM) blasts <10%, peripheral blasts <5%) and RAEB-2 (bone marrow blasts >10% and peripheral blasts >5%). We reclassified according to WHO criteria 228 RAEB patients and analysed them in terms of haematological, karyotypic and prognostic features. We used the database of 680 MDS patients referred to our Institution from 1990 to 2000. Clinical features at presentation, such as sex, age, leukocyte count, polymorphonuclear cell count (PMN), platelet count, haemoglobin level, presence of one or more lineage dysplasia were tested in univariate and multivariate analysis in the two groups of RAEB-1 and RAEB-2 reclassified patients. In multivariate analysis we identified prognostic significant factors in the two patient groups, which consisted of age >70 years and platelet count <100 x 10(9)l(-1) for RAEB-1 category, while for RAEB-2 group parameters negatively influencing survival and risk of progression were haemoglobin <10g/dl, platelet count <100 x 10(9)l(-1), bone marrow blastosis >15% and complex karyotype. We also found differences in cytogenetic data (more balanced translocations and complex karyotypes in RAEB-2 group, p=0.02), and in survival (23.3 months in RAEB-1 vs. 16.1 months in RAEB-2 group, p=0.001). WHO classification provides valuable prognostic information for RAEB patient population, and can identify those subjects with more unfavourable prognosis who should be offered alternative therapeutic strategies.