Neurofibromatosis type 1 is a common autosomal dominant genetic disorder associated with numerous physical anomalies and an increased incidence of neuropsychological impairment. Tumors of the CNS occur in approximately 15% of children with neurofibromatosis, presenting additional risk for cognitive impairment. This study examines the impact of an additional diagnosis of brain tumor on the cognitive profile of children with neurofibromatosis. A comprehensive battery of neuropsychological tests was administered to 149 children with neurofibromatosis. Thirty-six of these children had a codiagnosis of brain tumor. A subset of 36 children with neurofibromatosis alone was matched with the group of children diagnosed with neurofibromatosis and brain tumor. Although mean scores of the neurofibromatosis plus brain tumor group were, in general, lower than those of the neurofibromatosis alone group, these differences were not statistically significant. Children in the neurofibromatosis plus brain tumor group who received cranial irradiation (n = 9) demonstrated weaker academic abilities than did children with brain tumor who had not received that treatment. These results suggest that neurofibromatosis is associated with impairments in cognitive functioning, but the severity of the problems is not significantly exacerbated by the codiagnosis of a brain tumor unless treatment includes cranial irradiation.