Sirolimus is an immunosuppressive agent that was recently approved for use in organ transplantation. We propose to carry out a pilot study whose objectives are to determine the safety and efficacy profile of sirolimus in focal segmental glomerulosclerosis (FSGS). Current therapy for FSGS has limited efficacy. Sirolimus was selected for the following reasons: 1) sirolimus reduces proliferation of mesangial cells and endothelial cells, 2) sirolimus reduces fibrosis in experimental models of liver and kidney disease, 3) sirolimus is a potent immunosuppressive, and other immunosuppressives including glucocorticoids and cyclosporine have shown some efficacy in FSGS, and 4) sirolimus may have a direct anti-proteinuric effect, as suggested by in vitro studies. We will recruit up to 30 patients, including adults and children greater than or equal to 13.0 years of age. The study design is open label, with therapy for one year using doses adjusted to achieve trough levels of 5-15 ng/mL during the first 4 months and if a complete remission is not achieved and sustained, 10-20 ng/mL during the remainder of the study. The primary outcome will be reduction in proteinuria, categorized as complete remission and partial remission, comparing baseline values and 12 month values. The study will recruit patients in two groups: 1) a drug washout group, for patients who can tolerate receiving no immunosuppressive therapy for 4 weeks prior to initiating sirolimus therapy, and 2) a drug overlap group, for patients who cannot tolerate cessation of immunosuppressive therapy due to severe edema or other complications of nephrotic syndrome; these patients will receive prednisone for up to 6 months while taking sirolimus (with a target of prednisone less than 20 mg QOD by month 3) or will receive cyclosporine, tacrolimus, or mycophenolate mofetil for up to 4 weeks while initiating sirolimus therapy.