Patient participation is divided into an evaluation phase, in which FPF levels, RNA expression profiles, and patient eligibility for participation in treatment protocols are determined, and a treatment phase in which specific immunomodulatory therapy is introduced in an open label fashion. We propose to define carefully the relationship between elevated FPF and remission of proteinuria in patients with FSGS in native kidneys, following treatment with standard therapies (daily prednisone, cyclophosphamide) and experimental therapies (pulse dexamethasone, pirfenidone). In patients with recurrent FSGS in renal allografts, we will determine the kinetics of FPF following plasma exchange and following plasma exchange plus cyclophosphamide. In patients with elevated FPF levels who are awaiting renal transplantation, we will determine the kinetics of FPF following plasma exchange and following plasma exchange plus cyclophosphamide, and examine the rate of recurrent FSGS in these patients.