Eleven cases of renal vein thrombosis (RVT) are reported, 9 of which presented with nephrotic syndrome (NS) and 2 with acute renal failure (ARF). Hematuria, enlarged kidneys, assymetric peripheral edema and collateral vein dilatations were the classic symptoms. Phlebography was employed in 101 suspected cases and permitted definitive diagnosis in 11 percent. Phlebography was performed in 13 percent of the NS cases and 11 percent of the ARF cases, but in none of the other cases. Fifteen kidney biopsies (KB) were performed in 8 patients, 6 with NS and 2 with ARF. In the 6 cases of NS, 4 presented a pattern of membranous glomerulonephritis and 2 had mild thickening of the basement membrane. This was confirmed by electromicroscopy in 5. In the 2 with ARF capillary dilatations, polynuclear infiltrates and interstitial edema was found. Repetitive KB, performed in 3 cases, revealed the same histological pattern in 2, while in the other case granular membranous deposits were found on the second biopsy that were not apparent in the first. This last patient was free from clinical manifestations at the time of this last biopsy. Immunofluorescent studies (iF) were performed on 7 KB of 5 patients, 4 with NS and 1 with ARF. Three of the 4 with NS presented a pattern of membranous deposits. On i.F one patient was found to have IgG and C3 deposits, one IgG and fibrinogen and one no detectable IgG, IgA, IgM, IgE, C3 or fibrinogen. The fourth with minimal histological changes was also negative for i.F, as was the case of ARF. Sequential treatment by heparin and antivitamin K was administered in 8 patients. Recovery occurred in 3 and stabilization in 4, while in 1 case this treatment was without effect.