AbstractFabry's disease is an X‐linked recessive disorder of glycosphingolipid metabolism. Accumulation of neutral sphingolipid in vascular endothelium is known to cause various clinical manifestations, including cardiac problems. Female heterozygotes, however, may be asymptomatic or mildly affected. Cardiovascular manifestations in Fabry's disease were studied noninvasively in 10 male hemizygotes and 10 female heterozygotes from seven families. Mitral valve prolapse was found in both hemizygotes and heterozygotes. Measured echocardiographic variables were normalized by dividing them by normal values calculated from basal surface area (BSA) and age. Normalized thickness of the interventricular septum [IVS(n)] and left ventricular posterior wall thickness [LVPW(n)]increased significantly with age both in hemi‐ and heterozygotes. [IVS(n): Y=0.94+0.014X (age in years), r=0.68, LVPW(n): Y= 1.03+0.015X (age in years), r=0.64]. The incidence of ST‐T changes in the ECG was also observed in hemi‐ and heterozygotes. A short PR interval was recorded in one hemizygote and one heterozygote from the same family. Thus, heterozygotes appear not to be free from cardiac manifestations, as previously believed. It is possible that the cardiac changes in heterozygotes occur regardless of the amount of sphingolipid accum