Cardiovascular manifestations of Fabry's disease were studied clinically in 10 hemizygous males and 13 heterozygous females. Mitral valve prolapse was found in 5 of 9 hemizygotes and in 5 of 13 heterozygotes examined by echocardiography. Ordinary medical examinations revealed cardiomyopathy in some asymptomatic females, and the diagnosis of the Fabry heterozygote was established by demonstration of specific inclusion bodies in the biopsied myocardium and low alpha-galactosidase activity in leukocytes. Renovascular hypertension of juvenile onset and thromboembolism were also found in 7 patients. It was concluded that Fabry's disease should always be considered in cases of mitral valve prolapse, cardiomyopathy, renovascular hypertension and thrombosis of unknown etiology, and that the Fabry patients should be followed carefully for the early detection of cardiovascular involvements in this disease.