An obese patient with nephrotic syndrome was admitted to the hospital because of increasing edema in the legs. With 25 kg weight loss, proteinuria decreased from 15 g to 5 g/day. Renal biopsy revealed mesangial glomerulopathy. The serum IgE level was highly elevated, and the radioallergosorbent test (RAST) was strongly positive for many kinds of allergens. No significant change in proteinuria, compared with the highly right atrial pressure period, was observed after normalization in the right atrial pressure. In spite of a decrease in body weight (27 kg) (113 to 86 kg) in 140 days, no significant change in proteinuria was observed. After additional therapy with an anti-allergic drug, proteinuria was completely abolished. These results suggest that a combination of weight loss and treatment with an anti-allergic drug is very important therapy for massive obesity with nephrotic syndrome. Since RAST was strongly positive for many kinds of allergens, the pathophysiology in this nephrotic syndrome may be, at least partially, related to the immunologic abnormalities.