Purpose: The basis for cognitive problems in patients with neurofibromatosis type l (NF1) is unknown. A subset of NF1 patients with deletion of the entire NF1 gene has severe learning problems or mental retardation. We have reviewed neuroimaging studies (CT and MRI) in five such patients to determine whether structural anomalies in the brain are present and might explain the impaired cognitive function. Methods: Rve patients with NF1 and deletion of the entire gene were identified by FISH studies. A retrospective review was conducted of CT and MRI images, as well as of data from developmental assessments. Results: All five patients had severe developmental impairment. None had been exposed to chemotherapy or radiation therapy. All had multiple regions of bright T2 signal intensity. Structural anomalies were seen in three of the five patients and included callosal dysgenesis in one, septum cavum vergae and pellucidum in two, mega cisterna magna in one, and Chiari I malformation with severe hydrocephalus in one patient. Conclusion: Individuals with NF1 and large gene deletions have an increased frequency of structural anomalies of the brain not usually seen in NF1 patients. This suggests that the mental retardation in these individuals is due, at least in part, to abnormal brain devel-opment rather than a defect in brain function due to haplosufficiency of the NF1 gene product.