Bleeding manifestations are common in Gaucher disease patients. Although usually attributed to thrombocytopenia, some patients with relatively high platelet counts and normal coagulation tests have hemorrhagic phenomena. To investigate whether perturbed platelet function could explain these bleeding manifestations we performed platelet aggregation tests on 32 type I adult Gaucher patients who were not severely thrombocytopenic (platelet counts >50 x 10(9)/L). Seven patients (22%) had abnormal platelet aggregation. In five, platelet aggregation was markedly reduced in response to collagen and ADP and virtually absent in response to epinephrine, whereas two patients had isolated severely impaired epinephrine-induced aggregation. In one patient platelet aggregation markedly improved following one year of enzyme replacement therapy. Incubating normal platelets with high concentrations of glucocerebroside did not impair their ability to aggregate, suggesting that plasma glucocerebroside does not directly interfere with platelet function. Platelet dysfunction is a hitherto unrecognised, relatively common cause of excessive bleeding in Gaucher patients.