Forty-eight patients with nephrotic syndrome were evaluated prospectively; the studies included inferior venacavagrams and ventilation perfusion lung scans. Eleven patients were found to have renal vein thrombosis (RVT). Eight of 21 patients with membranous glomerulonephritis (MGN) or membranoproliferative glomerulonephritis (MPGN) has RVT (38%). Clinical, laboratory, and pathological findings were not different among those patients with MGN and MPGN whether RVT was present or not. Patients with diabetic nephropathy or lupus nephritis did not have RVT. There was a high incidence of other thromboembolic phenomena as well as asymptomatic perfusion defects demonstrated by the lung scan, especially in patients with MGN or MPGN. These data suggest the disease process underlying the nephrotic syndrome may play a paramount role in the genesis of RVT or thromboembolic phenomena.