A 58-year-old male patient had secondary polycythemia associated with idiopathic nephrotic syndrome. The renal biopsy revealed membranous nephropathy, and the bone marrow biopsy disclosed hypercellular marrow with mild panhyperplasia. The concentration of serum erythropoietin was 8.5 mU/ml. The erythrocytosis was characterized by an increased red cell volume (40.2 ml/kg) and normal arterial oxygen saturation. There were no associated lesions that could induce secondary polycythemia, except the biopsy-proven membranous nephropathy. He was treated with prednisolone and cyclophosphamide, and the nephrotic syndrome was partially remitted after 6 weeks. With partial remission of nephrotic syndrome, the erythrocytosis was resolved. This case illustrates the rarely reported association of the nephrotic syndrome and erythrocytosis, and the resolution of erythrocytosis with improvement of nephrotic syndrome.