A 22 -year-old white man was found to have a serum creatinine level of 3.4 mg/dL (259 micromol/L) and 6 g of protein in a 24-hour urine collection. Laboratory studies performed 5 years prior showed no evidence of abnormal renal function. Neurofibromatosis had been diagnosed in infancy, and genetic testing showed that the patient had a mutation in chromosome 17, consistent with neurofibromatosis type I. Mutations on chromosomes 1, 11, and 19 have been reported to result in genetic forms of focal segmental glomerular sclerosis. This is the first case report of focal segmental glomerular sclerosis occurring in a patient with neurofibromatosis type I.