Platelet mass (mean platelet volume x platelet count) can be derived from data obtained from the routine full blood count and separates patients with myelodysplastic syndromes (MDS) at diagnosis into three distinct prognostic groups: low platelet mass group - median survival 5 months and 5-year survival 0%; intermediate platelet mass group - median survival 30 months and 5-year survival 34%; high platelet mass group median survival - not reached at 82 months follow-up with a 5-year survival of 82%. These data provide a simple rapid prognostic index at the time of diagnosis in MDS.