Neurofibromatosis type 1 (NF1) has a very heterogeneous phenotype. It is not currently possible to predict which patients will have mild disease and which will develop serious complications. Medical management of patients with NF1 might be improved if subgroups of patients who are at especially high (or low) risk for particular complications could be identified. We have begun an analysis of NF1 patients in the National Neurofibromatosis Foundation International Database (NNFFID) to identify possible associations between the occurrence of clinical features. A striking association has been observed between the presence of optic glioma and of other central nervous system (CNS) tumours in NF1 patients. This association is not dependent on the effect of age. No association is seen between optic glioma and non-CNS neoplasms. The association of optic glioma and other intracranial neoplasms in patients with NF1 suggests that there are fundamental pathophysiological differences between patients with and without optic glioma.