The objective of this pilot phase II trial is to evaluate the ability of pirfenidone, a novel anti-fibrotic agent, to reduce the proteinuria and slow the rate of progression of renal insufficiency in patients with focal segmental glomerulosclerosis (FSGS). We will enroll 25 patients with renal biopsy proven FSGS and evidence of impaired renal function (glomerular filtration rate, GFR, of 10-80 ml/min; after 1/02 must have GFR greater than 25 ml/min) as assessed by the 4 variable Modification of Diet in Renal Disease equation. As standard of care therapy, all patients will also receive angiotensin converting enzyme inhibitor (ACEI) therapy, and will receive an HMG Co-A reductase inhibitor drug if hypercholesterolemic. Preliminary evaluation will assure that the patients meet the study requirements, and an evaluation period will be used to ensure that patients are on maximal conservative therapy prior to the baseline period. Patients will receive treatment with pirfenidone daily, with dose adjusted for body weight and level of kidney function. The primary end point will be the decrease glomerular filtration as a marker of glomerular injury; reduction in proteinuria will be a secondary end-point. If the pilot study suggests this drug delays progression of renal insufficiency or reduces proteinuria in patients with FSGS, we will proceed with a large scale randomized, placebo-controlled study.