A patient with an abdominal mass was found to have a Wilms' tumor. Light microscopic examination showed that, in addition to blastema, stroma, and tubules, the tumor contained an unusually large number of glomeruli. Glomerular basement membranes stained positively by immunohistochemistry for laminin, collagen type IV, fibronectin, and Goodpasture antigen. Staining for Goodpasture antigen was seen only after acid-urea treatment, which was similar to findings in fetal and infant glomeruli. Glomerular cells stained positively for actin, myosin, and desmin, but there was no staining for factor VIII or Ulex europaeus agglutinin I, indicating an absence of endothelial cells. These findings were supported by electron microscopy, which showed basement membrane material, visceral epithelial cells, and mesenchymal cells (presumably primitive mesangial cells) in glomeruli, but no patent capillaries or capillary endothelium. Hence, the glomeruli in this case of Wilms' tumor were immature and also showed aberrant glomerulogenesis.