Children with the neurofibromatosis type 1 (NF1) inherited tumor predisposition syndrome are at risk for the development of brain tumors. In addition, children with neurofibromatosis type 1 often exhibit low tone (hypotonia). In this study, the authors explored the hypothesis that hypotonia could be a clinical indicator of glioma in children with neurofibromatosis type 1. A total of 56 children between 1 and 7 years of age with a confirmed diagnosis of neurofibromatosis type 1 were evaluated. Brain magnetic resonance imaging (MRI) was available for 19 of these children. Chi-square analysis demonstrated a statistically significant correlation between hypotonia and glioma in children with neurofibromatosis type 1 (90% sensitivity and 78% specificity). These results suggest that hypotonia might be a clinically useful indicator of brain tumor in this at-risk population.