Forty-eight patients with myelodysplastic syndromes and a platelet count greater than 80 x 10(9)/L were the subjects of a study of platelet function. A whole-blood platelet lumi-aggregometer was used for simultaneous measurement of platelet aggregation by the impedance method and of adenosine triphosphate-dense granule release. The results were correlated with skin bleeding time and episodes of clinical bleeding or thrombosis. Thirty-five patients had at least 1 abnormal result indicating platelet hypoactivity; 7 patients had mixed platelet hypoactivity and hyperactivity; and 4 patients had platelet hyperactivity. Only 2 patients had normal results. There was good correlation between platelet hypoactivity and prolonged skin bleeding time (P = .005); however, several patients with platelet hypoactivity had normal skin bleeding times. This finding suggested that whole-blood platelet aggregation studies may be more sensitive than bleeding time in identification of patients at risk of bleeding. Clinical hemorrhage was frequent (32 patients) in this cohort despite platelet counts greater than 100 x 10(9)/L. This finding indicated platelet hypofunction was clinically important. In contrast, only 2 of the 13 patients with thrombotic events had evidence of platelet hyperactivity, suggesting that other clinical factors are probably more important determinants of thrombosis. These observations confirm that platelet dysfunction is common in patients with myelodysplastic syndromes and suggest a useful role for routine whole-blood platelet aggregation studies to identify patients at risk of bleeding.