OBJECTIVE: People with neurofibromatosis type 1 (NF1) have a 10% lifetime risk of developing a malignant peripheral nerve sheath tumor (MPNST). MPNSTs are often metastatic and are a frequent cause of death among people with NF1. Clinical evidence suggests that most MPNSTs in people with NF1 develop from preexisting plexiform neurofibromas. However, it is not known whether an individual's risk of developing an MPNST is associated with the burden of benign neurofibromas. The authors conducted a study to determine whether people with NF1 who have benign neurofibromas of various kinds are at greater risk of developing MPNSTs than patients with NF1 who lack these benign tumors. METHODS: Clinical information on 476 NF1 probands in the Henri Mondor Database was analyzed by logistic regression to examine associations between MPNSTs and internal plexiform, superficial plexiform, subcutaneous, and cutaneous neurofibromas. RESULTS: Individuals with subcutaneous neurofibromas were approximately three times more likely to have internal plexiform neurofibromas or MPNSTs than individuals without subcutaneous neurofibromas. Individuals with internal plexiform neurofibromas were 20 times more likely to have MPNSTs than individuals without internal plexiform neurofibromas. When this analysis was done with both subcutaneous and internal plexiform neurofibromas as explanatory variables, only the association of MPNSTs with internal plexiform neurofibromas remained significant. CONCLUSIONS: The observation that malignant peripheral nerve sheath tumors are strongly associated with internal plexiform neurofibromas suggests that patients with neurofibromatosis type 1 with these benign tumors warrant increased surveillance for malignancy.