Intellectual problems are a recognized feature of neurofibromatosis 1 (NF1) but their aetiology is unknown. We investigated the frequency, nature, severity and cause of intellectual impairment in NF1. We undertook neurological and psychometric assessments on 103 patients with NF1 and 105 controls equated for age, sex and socio-economic status. The mean full scale IQ was significantly lower in the NF1 than the control group, 88.6 (SD 14.6) compared with 101.6 (SD 14.2). However, the degree of intellectual impairment was mild and only 8% of NF1 patients had an IQ < 70. The NF1 patients also had significantly poorer reading skills and impaired short term memory. On a computerized performance test battery of complex tasks, the NF1 group had significantly slower mean reaction times and higher error rates than the controls. Overall the patients displayed impaired attention and were slow to develop and adapt strategies for complex and unfamiliar tasks. A particular profile of intellectual deficit did not emerge. The presence of neurological and/or medical complications was weakly associated with a lower mean full scale IQ in NF1 patients. Socio-demographic factors, age or sex differences and the presence of macrocephaly did not contribute to neurocognitive deficit in NF1.