A 55 year old woman with heterozygous Fabry's disease presented with cardiac symptoms. The electrocardiogram showed a PR interval of 0.12 s and giant negative T waves, suggesting apical hypertrophic cardiomyopathy. Endomyocardial biopsy, however, revealed myelin like substances characteristic of Fabry's disease. Increasing thickness of the left ventricular wall was seen by echocardiography over a period of five years. A deficiency of alpha galactosidase activity in the leucocytes confirmed the diagnosis of Fabry's disease, although this patient had neither angiokeratoma or proteinuria. The possibility of Fabry's disease should be considered in patients with cardiomegaly of unknown cause and the following electrocardiographic abnormalities: a PR interval less than or equal to 0.12 s, high voltage QRS complexes in the left precordial leads, and giant negative T waves.