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.