 This study will evaluate the safety and effectiveness of a new immunosuppressive drug, sirolimus, in reducing the amount of protein in the urine in patients with membranous nephropathy. This condition involves damage to the walls of tiny blood vessel filters in the kidneys called glomeruli, which allows blood proteins to leak into the urine. Patients have low blood protein levels and high blood cholesterol. Some patients may have leg swelling, impaired kidney function, blood vessel and heart disease, and a risk of emboli blood clots that travel to the lungs. Drugs currently used to treat membranous nephropathy vary in their effectiveness among patients and can cause severe side effects. 