43 patients and 27 relatives suffering from a hypertrophic type of peroneal muscular atrophy have been examined. Following the classification of peroneal muscular atrophy by Dyck and Lambert early age of onset and considerable reduction of nerve conduction velocity served as criteria for allotting them to the hypertrophic type. In all patients a typical deformity of the foot developed before the age of 10. Among the relatives there were some without this deformity and with only slight clinical signs. Assessing consistency and thickness of peripheral nerves by palpation proved difficult and unreliable. These findings also varied considerably between members of one family. The motor nerve conduction velocity was less than one half of the normal on average. Even those patients with only minor clinical signs had a nerve conduction velocity reduced by one fifth. In about 60% of the patients there was dominant inheritance, in 3 it was recessive and in about one third nothing was known. 10 of the patients without dominant inheritance had rather severe neurologic disturbance and a kyphoscoliosis. Whether this is hypertrophic neuritis of Dejerine und Sottas is discussed. Data form the literature differ. Another common type of peroneal muscular atrophy is the neuronal type. In contrast to the hypertrophic type it generally begins in adults and nerve conduction velocity is hardly affected.