A patient with hairy cell leukemia developed purpura not attributable to thrombocytopenia. We found markedly reduced platelet aggregation responses and malondialdehyde production, decreased serotonin uptake, and depleted dense granule contents. Ultrastructural studies showed that most platelets had few or nor granules. All of the clinical and laboratory studies of platelet function and morphology improved after splenectomy. These findings indicate that qualitative defects in platelet function occurring in hairy cell leukemia may cause clinically important bleeding and that the bleeding diathesis may be ameliorated by splenectomy.