This study will examine the effectiveness of the drug pirfenidone in treating focal segmental glomerulosclerosis (FSGS). Patients with this disease have kidney fibrosis (scarring) and proteinuria (excessive excretion of protein in the urine). About half of patients with FSGS eventually require kidney dialysis or transplant. Steroids, which are currently used to treat the disease, are effective in only a minority of patients. Other drugs, such as cyclosporin and cyclophosphamide, improve proteinuria in a very small percentage of patients and have serious side effects.