Three adolescents with unilateral renal vein thrombosis and the nephrotic syndrome are presented. Light, fluorescent, and electron microscopic examination of bilateral renal biopsy specimens in each patient demonstrated membranous glomerulonephritis in both kidneys. In the first case there was no evidence of preexisting renal disease and the initial biopsy at the time of thrombosis revealed near normal glomeruli on light microscopy. Fluorescent staining and the electron microscope demonstrated immune complex deposition along the glomerular basement membrane. Follow-up biopsy of the same case seven months later showed advanced membranous glomerubonephritis on light and electron microscopy. These cases as well as data from recent animal experiments raise the interesting suggestion that renal vein thrombosis and the nephrotic syndrome might be an immunologic disease. Unilateral obstruction of the renal vein may result in renal antigen release and antigen-antibody complex formation with subsequent glomerular deposition, bilateral glomerular disease, and the nephrotic syndrome.