Background: Neurofibromatosis Type 1 (NF1) is an autosomal dominant, progressive genetic disorder characterized by diverse clinical manifestations. Patients with NF1 have an increased risk of developing tumors of the central and peripheral nervous system including plexiform neurofibromas, which are benign nerve sheath tumors that may cause severe morbidity and possible mortality. The histopathology of these tumors suggests that events connected with formation of fibroblasts might constitute a point of molecular vulnerability. Gene profile analysis demonstrates overexpression of fibroblast growth factor, epidermal growth factor, and platelet-derived growth factor in plexiform neurofibromas in patients with NF1. Pirfenidone is a novel antifibrotic agent that inhibits these and other growth factors. Clinical experience in adults has demonstrated that pirfenidone is effective in a variety of fibrosing conditions and pirfenidone is presently under study in a phase II trial for adults with progressive plexiform neurofibromas. A phase I trial of pirfenidone in children and young adults with NF1 and plexiform neurofibromas was completed, and has established the phase II dose (the dose resulting in a mean drug exposure \[AUC\] not more than 1 standard deviation below the mean drug exposure \[AUC\] in adults who received pirfenidone at the dose level demonstrating activity in fibrosing conditions). Pirfenidone has been well tolerated. Objectives: To determine whether pirfenidone increases the time to disease progression based on volumetric measurements in children and young adults with NF1 and growing plexiform neurofibromas. To define the objective response rate to pirfenidone in NF1-related plexiform neurofibromas. To describe and define the toxicities of pirfenidone. Eligibility: Individuals (greater than or equal to 3 years to less than or equal to 21 years of age) with a clinical diagnosis of NF1 and inoperable, measurable, and progressive plexiform neurofibromas that have the potential to cause substantial morbidity. Design: The phase II dose will be used in a single stage, single arm phase II trial The natural history of the growth of plexiform neurofibromas is unknown. For this reason, time to disease progression on the placebo arm of an ongoing National Cancer Institute (NCI) Pediatric Oncology Branch (POB) placebo-controlled, double-blind, cross-over phase II trial of the farnesyltransferase inhibitor R115777 for children and young adults with NF1 and progressive plexiform neurofibromas. Funding source - Food and Drug Administration (FDA) Office of Orphan Products Development (OOPD)