A population-based study in south-east Wales (population 668,100) identified 135 patients with von Recklinghausen neurofibromatosis (prevalence 20/10(5]. In addition to multiple café-au-lait spots and/or dermal neurofibromas, freckling was present in the axilla (67%), groin (44%) or submammary areas (29% of adult females). Although not a criterion for diagnosis, Lisch nodules were almost invariably present in the iris (93% of patients overall; 96% of those aged greater than or equal to 20 yrs). The complications of von Recklinghausen neurofibromatosis in this cohort (n = 135 unless stated) were plexiform neurofibromas (40/125), severe mental retardation (1), epilepsy (6), optic glioma (2), spinal neurofibroma (2), aqueduct stenosis (2), meningioangiomatosis (1), scoliosis requiring surgery (6), pseudoarthrosis (3), delayed puberty (2), visceral and endocrine tumours (6), and congenital glaucoma (1). There were no cases of acoustic neuroma. Considering all living family members aged greater than or equal to 18 yrs, together with their deceased relatives, the frequency of CNS and malignant tumours related to the disease was 4.4-5.2%. Uncomplicated von Recklinghausen neurofibromatosis is disfiguring but not a major cause of morbidity. The management of the disease relates to its complications which can be divided into three categories: those which occur in childhood and cause lifelong morbidity (moderate-severe mental handicap, facial plexiform neurofibromas, orthopaedic), those which can occur at any time but are 'treatable' (benign disorders of the nervous system, visceral and endocrine tumours, renal artery stenosis), and malignant or CNS tumours. The combined frequency for each category based on this survey was 12%, 16% and 4.4-5.2%, respectively.