Editor—Neurofibromatosis type 1 (NF1), also known as von Recklinghausen's disease, is an autosomal dominant neurocutaneous disease characterised by café au lait spots and neurofibromas. The gene responsible for the disorder is located in the chromosome region 17q11.2. The prevalence of NF1 has been estimated to be about 1/3500 in the USA and the United Kingdom,1-3 and its birth incidence has been reported to vary from 1/25584to 1/4292,5 the mutation rate being 3.1 × 10^–5 to 6.5 × 10^–5. The first population based study of NF1 was performed in Sweden by Samuelsson6in 1981, who found 74 adult NF1 patients in the Gothenburg region, implying a prevalence of 1/4600. In 1989, Huson et al 4 discovered 135 NF1 patients in 69 families in south east Wales, 83 of whom were index cases and 52 affected relatives, with a prevalence of 1/4150. The highest estimated prevalence for NF1, 1/2190, has been reported in Dunedin, New Zealand, by Fuller et al,7 who also showed that the prevalence peaked in the age group 20-29 years. A fourth report from north east Italy by Clementi et al 5 quoted a prevalence of 1/6711 and a very high mutation rate of 6.5 × 10^–5 gametes per generation.

The purpose of the present work was to determine the prevalence and genetic characteristics of NF1 in northern Finland, including a survey of first degree relatives of patients and linkage data, to assist in the diagnosis of affected subjects. Clinical data on the patients will be reported separately.

The study was carried out between October 1989 and December 1996 in the region of Oulu University Hospital (OUH) in northern Finland with a total population of 733 037 (31 December 1996).8The basic material consisted of families …