We report a family with Noonan syndrome (NS), giant proximal nerve hypertrophy, and hereditary motor sensory neuropathy type 1A (HMSN1A). Five members of a family were found to have clinical features of NS. In all cases, NS was associated with giant hypertrophy of proximal nerves and two individuals also exhibited café-au-lait spots. In one case, an 8-to-10-cm diameter pelvic mass was shown to be a grossly hypertrophied nerve, with histologic features of demyelination and remyelination. In addition, four of five family members affected with NS were found to have HMSN1A clinically and by demonstration of constitutional HMSN1A duplication on DNA testing. Linkage analysis for NS ruled out the involvement of the neurofibromatosis type 1 gene and the known NS locus in chromosome 12, supporting the existence of an additional NS locus.