Ninety-six persons, living in Gothenburg, Sweden, on the first of January 1978 and known by the health services as cases of neurofibromatosis, were investigated concerning clinical and genetic aspects of the disease. Close relatives were also interviewed and examined. The diagnostic criteria were operationally defined according to number of café-au-lait spots and/or neurofibromas. The prevalence of neurofibromatosis was estimated to be 0.02%. The following findings were made in the patients: axillary freckling (48%), neurological symptoms (30%), epilepsy (3-9%), sarcoma (4%), pheochromocytoma (3%), osseous dysplasias (12%), subnormal intelligence (45%) and psychiatric symptoms (33%). The genetic analysis revealed a dominantly inherited disease with full penetrance and a very high mutation frequency, at least 4.3 X 10(-5). Questions commonly encountered during counselling are discussed.