The morphologic features observed in the renal biopsy specimens from three patients with massive obesity complicated by the nephrotic syndrome are described. In the two patients with active disease, the majority of the glomeruli showed focal to extensive fibrin deposition both within and adjacent to capillary loops, associated with adhesions to Bowman's capsule, with a variable endocapillary proliferative response. In the patient with inactive disease, the majority of the glomeruli appeared normal. However, fibrin-like material, similar to that seen in the active phase, was infrequently found ultrastructurally in the subendothelial portion of the capillary loops. Subendothelial vacuolated material was also observed. Although intravascular coagulation was suggested in this disorder, only minor coagulation abnormalities were discovered. In two of the patients, the massive proteinuria has resolved with weight reduction.