This study will determine the safety and effectiveness of sirolimus (Rapamune® (Registered Trademark)) in treating focal segmental glomerulosclerosis (FSGS), a disease involving kidney scarring and increased protein in the urine. About one-half of patients with FSGS go on to develop end-stage kidney disease within 6 years, requiring dialysis or kidney transplant. Therapies to reduce urine protein are likely to stop the progression of renal scarring and reduce the chance of developing kidney failure. However, current treatments for FSGS, such as prednisone, cyclophosphamide, and cyclosporine, are not effective in many patients and can cause serious side effects. This study will see if sirolimus, a drug with both anti-scarring and immune suppressing properties, can lower the amount of protein in the urine and slow or stop the kidney disease.