During 2 years we studied 36 of 41 consecutive patients with nephrotic syndrome. Twelve (Group 1) had renal vein thrombosis; the renal pathology was membranous glomerulonephritis in 10 and membranoproliferative glomerulonephritis in 2. Eighteen without renal vein thrombosis (Group 2) had either membranous (10 patients) or membranoproliferative (8 patients) glomerulonephritis. Clinical presentation, laboratory data, renal function, renal histology, and immunopathology were similar in both groups; thromboembolic phenomena were frequent. Ten of 12 patients with renal vein thrombosis were given oral anticoagulants and followed for 4 to 24 months. None died or had thromboembolic episodes. The results show that one third of the patients with nephrotic syndrome associated with either membranous or membranoproliferative glomerulonephritis had renal vein thrombosis, and there were no clinical, laboratory, histological, or immunopathological finding(s) diagnostic of renal vein thrombosis. Our data indicate that renal vein thrombosis is a frequent complication in nephrotic syndrome and membranous or membranoproliferative glomerulonephritis.