This report describes the light, electron, and fluorescence microscopy studies of the kidney from a one month old, prematurely born infant with the nephrotic syndrome. Light microscopy revealed enlargement of Bowman's space, an excessive number of glomerular epithelial cell nuclei, and cystic dilatation of cortical tubules. By electron microscopy two disparate types of glomerular lesions were found: a thin basement membrane with fusion of foot processes suggestive of lipoid nephrosis and linear electron-density in the basement membrane with excessive mesangial matrix and capillary collapse suggesting an antibasement membrane type of glomerulonephritis or mesangial sclerosis. Conspicuous cystic spaces were found in the proximal tubules only. In the arterioles there were cystic spaces in the endothelial cells and smooth muscle cells, thickening of the basement membranes of endothelial cells and smooth muscle cells, and excessive amounts of nonstriated fibrils in the subendothelial basement membrane. Masses of fibrin were noted in the lumina of veins, peritubular capillaries, and interstitium between veins and peritubular capillaries. Such multiple glomerular and arteriolar lesions have not been reported previously in congenital nephrotic syndrome. These lesions suggest an antigen-antibody reaction in the mother that might have been directed toward the glomerular basement membrane in the fetus, producing these lesions.