Vacuoles in glomerular visceral epithelial cells were found in 149 of 254 patients with idiopathic membranous glomerulonephritis (IMGN). In the whole population studied, 115 patients were nephrotic, 88 (76.5%) of which were found to have intraepithelial vacuoles. Fifty (35.9%) of 139 non-nephrotic patients had vacuolated podocytes (p < 0.01). Vacuolation occurred most frequently in stages II and III of IMGN. A few intraepithelial vacuoles were observed in stage IV. Increase in vacuolation tended to correlate with subepithelial deposits which were larger in size. From these results, we concluded that the appearance of epithelial vacuolation coincides with an increased filtration of protein in IMGN and that it is an important histological marker when diagnosing the level of severity of glomerular lesions. It was not, however, prognostic.