In this study, a clinical evaluation was carried out for all patients who were at or beyond 20 years of age and known to the health services as cases of neurofibromatosis (NF) and who were resident in Gothenburg, Sweden, as of January 1, 1978. The approach of the study was clinical, with emphasis on the general somatic, psychiatric and genetic aspects of NF. The patients included in the study were ascertained by scrutiny of all available archives of medical records in the area, and by requests to every doctor in the city of Gothenburg to report any possible case of NF known to him or her. This search identified 74 patients with NF living in Gothenburg on the census day. All but 3 of these patients had definite von Recklinghausen NF (NF-1). This represents a prevalence of 1 case of NF in 4,600 adults, which must be considered a minimum frequency estimate. The 74 patients included 35 women with a mean age of 46 (+/- 17) years and 39 men with a mean age of 43 (+/- 14) years. The prevalence of NF was highest in the age range of 40-50 years, while it was significantly reduced in the ages above this range, most probably owing to an excess mortality. Sixty-nine of the original seventy-four patients were personally interviewed and examined, including the 3 without definite NF-1. The patients were classified according to the degree of severity of NF into three categories: mild, moderate and severe. The number of patients in each groups was, respectively, 18, 43 and 13. A detailed description of each patient's pigmentary abnormalities and neurofibromas (number, appearance and localization) was recorded. Findings of osseous dysplastic changes (12-16%), endocrine changes (pheochromocytoma, 3%), malignant disease (sarcoma, 4%), epilepsy (3%) and other somatic diseases were also recorded. Mild mental retardation was present in 45% of the patients. The mental retardation did not appear progressive, and severe retardation was not found. Mental illness occurred in 23 (33%) patients, defined as mild in 8, moderate in 7 and severe in 8. No uniform psychiatric syndrome was found. Depressive syndrome, anxiety state with vegetative dysfunction and organic brain syndrome were most frequently observed. Hostile feelings and autonomic disturbances were the most common symptoms, each found in 50% of all patients. The frequency of abnormal neurological findings, presumably indicating manifestations of NF-1 in the central nervous system, was significantly increased among the patients with mental illness.(ABSTRACT TRUNCATED AT 400 WORDS)