A patient with carcinoma of the prostate and metastases developed the nephrotic syndrome following hormonal therapy. A decrease in the dose of estrogens was associated with decreased proteinuria, but when therapy was increased the nephrotic syndrome became more severe. Renal biopsy performed when proteinuria was present showed subendothelial electron dense deposits, complement by immunofluorescence, and a morphologic pattern of membranoproliferative glomerulonephritis. There was evidence of resorption of the deposits by endothelial cells. Microspherical particles resembling those described in prostatic cancer were found in the glomeruli. On the basis of previous reports it is concluded that circulating tumor antigen-antibody complexes produced glomerulonephritis, the severity of which was related to the amount of soluble antigen released by tumor cell destruction. Apparent phagocytosis of immune complexes by glomerular endothelial cells was believed to account for the reversibility of the nephrotic syndrome. The role of the virus-like particles in the process is as yet unclear.