The study of platelet function in thirty-one patients with left ventricular hypertrophy of various origins (9 cases of hypertensive cardiomyopathy, 11 of hypertrophic obstructive cardiomyopathy and 11 of aortic stenosis) revealed low serotonin content of platelets on the patients with left ventricular tract obstruction but normal levels in the hypertensive cardiomyopathy group. This is attributed to platelet activation induced by important flow disturbances related to the hemodynamic conditions.