Two unrelated children with von Recklinghausen's neurofibromatosis (NF1) had mothers with cutaneous NF1 lesions in a limited distribution. The cutaneous pattern in the mother of case 1 was clearly segmental, and probably represents mosaicism for the NF1 mutation which was passed on to the child. In the second case the distribution in the mother was less obviously segmental, but may still represent mosaicism. It is more difficult to diagnose mosaicism for NF1 in individuals with no affected offspring, or with more limited cutaneous manifestations. The difficulties in defining segmental NF and assigning a genetic risk are discussed.