Neurofibromatosis type 1 (NF1) is an autosomal dominant disease with a prevalence of about 1/3000. The clinical diagnosis of NF1 is based on the presence of two or more of the following criteria: six or more café au lait spots, >2 neurofibromas of any type or 1 plexiform neurofibroma, freckling in the axillary or inguinal region, optic glioma, a distinctive osseous lesion such as sphenoid dysplasia or thinning of long bone cortex with or without pseudoarthrosis, and a first degree relative with NF1. The disease has numerous complications. The pathogenesis is not clarified. The NF1 gene is a megagene (length of app. 350 kilobases of genomic DNA), localised on the long arm of the 17th chromosome at the position 17q11.2. The mutation rate for NF1-gene is high. The half of all cases of NF1 are from new mutations. The gene protein product - neurofibromin plays an important role in the tumorogenesis as a tumor-suppressor gene. Molecular genetic evaluation for the identification of disease-causing mutations is possible in cca 20% patients. The patient care is best done in specialised neurofibromatosis centers.