Neurofibromatosis type 1 (NF1) is an autosomal dominant disorder whose major feature is the occurrence of multiple neurofibromas, which are benign tumors of the nerve sheath. It affects an estimated one in 3000 to 4000 individuals. In addition to neurofibromas, there are many other clinical manifestations, including malignant tumors such as gliomas or malignant peripheral nerve sheath tumors, and nontumor effects such as skeletal dysplasia and learning disability. Diagnosis is established on the basis of clinical criteria. Molecular genetic testing is feasible, but the large size of the gene and wide range of pathogenic mutations have so far impeded the development of a clinical diagnostic test. Insights into pathogenesis have followed from identification of the NF1 gene and the development of animal models. The major function of the gene product appears to be regulation of the ras protein. Tumors are believed to arise by the loss of function of the NF1 protein, suggesting that NF1 behaves as a tumor suppressor gene. Heterozygous effects on some cell types are also likely, however. The role of ras in the pathogenesis of tumors in NF1 has suggested an approach to treatment using ras inhibitors, some of which are likely to begin in clinical trials in NF1 patients in the near future.