BACKGROUND AND OBJECTIVES: The aim of this study was a prospective validation of the World Health Organization (WHO) proposals for the classification of myelodysplastic syndromes (MDS) with respect to their prognostic relevance. DESIGN AND METHODS: We classified 1095 patients with MDS diagnosed at our institution between November 1999 and December 2004 according to French-American-British (FAB) and WHO criteria by central morphologic review. The study was not population-based, but included all newly diagnosed patients from different regions in Germany. Patients were followed for survival and disease evolution to acute myeloid leukemia (AML) through December 31th, 2005. RESULTS: According to the WHO classification, there were 89 cases of refractory anemia (RA), 293 of refractory cytopenias with multilineage dysplasia (RCMD), 31 RA with ringed sideroblasts (RARS), 139 RCMD with ringed sideroblasts (RCMD-RS), 142 RA with excess blasts (RAEB) I and 149 RAEB II and 52 patients with 5q- syndrome. The median survival of patients with RA or RARS was not reached, the median survival of patients with RCMD was 31 months, that of patients with RCMD-RS was 28 months, that of 5q- patients was 40 months, of RAEB I 27 months and of RAEB II 12 months. The cumulative risk of AML evolution 2 years after diagnosis was 0% in RA and RARS, 8% in 5q-, 9% in RCMD, 12% in RCMD-RS, 13% in RAEB I and 40% in RAEB II. The number of high-risk karyotypes was lower in patients with RA/RARS than in those with RCMD/RCMD-RS and RAEB I/RAEB II. Karyotype findings were major prognostic variables. INTERPRETATION AND CONCLUSIONS: The WHO classification is feasible and provides valuable prognostic information, even in a short-term prospective study. Together with cytogenetic data and other prognostic parameters, the WHO classification is very useful for clinical decision making.