Systematic electron microscopic investigation of glomeruli of 35 patients with renal amyloidosis (grade I-III), among them 26 with nephrotic syndrome, reveals the following: 1. The extent of the area of basement membrane denuded of its epithelial covering is correlated significantly with the reduction of plasma protein concentration at the time of renal biopsy. 2. In amyloid free regions of the glomerular capillary loops, the foot processes of the epithelial cells remain intact despite the presence of the nephrotic syndrome. From these findings we conclude that the high glomerular protein losses in amyloidosis occur in areas of the basement membrane which are penetrated by amyloid and denuded of their epithelial covering. With increasing number of these lesions per unit area, the permeability of the capillary network for protein increases to a degree which is significantly correlated with the reduced plasma protein concentration at the time of biopsy.