THIS report gives the neurologic, genetic, and electrophysiologic findings of a prospective study of kinships with different hereditary neurologic disorders having symmetric neurogenic weakness and atrophy as an early- and often major manifestation. The study was undertaken to obtain more reliable information about the natural history of these disorders, to test the usefulness of electro physiologic studies in distinguishing affected from nonaffected persons, to compare the results of nerve biopsies from representative affected persons with these disorders, and to develop a more meaningful classification. In this, the first of two articles on the subject, the results of neurologic, genetic, and electrophysiologic studies of 67 persons with hypertrophic neuropathy of the Charcot-Marie-Tooth type, five persons with hypertrophic neuropathy of the Dejerine-Sottas type, and 150 unaffected relatives are given. We also wished to determine whether these syndromes were phenotypic variations of one or several diseases or were separate entities on the