Neurofibromatosis type 1 (NF 1)--morbus von Recklinghausen is an autosomal dominant phacomatosis with variable expression. The gene for NF 1 is located on chromosome 17q11.2. Incidence is 1 in 3500 live births. The diagnosis is made on the basis of clinical manifestations. Diagnosis requires the presence of 2 or more major criteria: 6 or more café au lait spots, 2 or more cutaneous neurofibromas or 1 plexiform neurofibroma, an optic nerve glioma, 2 or more iris Lisch nodules, axillary or inguinal freckling, bony lesions--pseudoarthrosis, sphenoid wing hypoplasia, or a first-degree relative with NF 1.