In men with classical Fabry disease (alpha-galactosidase A [alpha-Gal A] deficiency), kidney failure occurs as early as the second decade of life. In contrast, men with the mild "cardiac variant" have late-onset cardiac involvement and proteinuria but usually do not have renal failure. To investigate the nature of renal involvement in the cardiac variant of Fabry disease, the renal function and morphology were assessed in a 75-year-old affected man. He had mild congestive heart failure, a reduced left ventricular ejection fraction, and hypercholesterolemia but lacked the classical Fabry disease manifestations, including angiokeratoma, acroparesthesias, corneal and lenticular opacities, and hypohidrosis. At age 75 years, he had significant proteinuria, and mildly decreased renal function (serum creatinine, 1.8 mg/dL [159 micromol/L]), presumably secondary to hypertensive arteriosclerosis. He had about 4% residual alpha-Gal A activity in leukocytes, and mutation analysis identified the N215S missense mutation, the common lesion in cardiac variants. Histologic and ultrastructural studies of kidney tissue showed that lysosomal glycosphingolipid deposition was extensive in podocytes, rare in tubular epithelial cells, and absent in mesangial, interstitial, and vascular endothelial and smooth muscle cells. This cardiac variant serves as an "experiment of nature" showing that the residual alpha-Gal A activity precludes glycosphingolipid deposition in the renal endothelial and other cells that lead to early renal failure in classically affected men, whereas marked podocyte accumulation is associated with proteinuria and possibly late-onset renal dysfunction. These findings have important implications for the renal effectiveness of enzyme replacement therapy in classically affected patients and for the aggressive treatment of proteinuria in Fabry disease.