Individuals with the autosomal dominant genetic disorder, neurofibromatosis (NF), are thought to be at increased risk of cancer, particularly neural tumors. Multidisciplinary study of individuals with this condition was undertaken (1) to seek a reliable biomarker for NF, (2) to use genetic linkage analysis to localize the NF gene, (3) to quantify more precisely the frequency and pattern of cancers associated with NF, and (4) to evaluate selected patients and families with NF (e.g., twins, homozygotes) to learn about factors which influence the clinical heterogeneity of the condition and its link with cancer. Patients with this disorder were evaluated in the Inter-institute Genetics Clinic, and on field trips. In addition, data on affected individuals were provided by the National Neurofibromatosis Foundation and the Late Effects Study Group.