Forty-three patients with hereditary motor system diseases belonging to 17 familes were studied. The clinical features consisted of a bilateral pyramidal syndrome, weakness with atrophy and fasciculation of the hands and/or the legs, with or without a bulbar or a pseudobulbar syndrome and without sensory disturbance. Electromyography in 31 cases (including all index cases) showed evidence of denervation.Motor and sensory nerve conduction velocity was normal; sensory nerve action potential amplitudes, examined in 11 cases, were also normal. Nerve and muscle biopsies taken in 29 cases (including all index cases)showed neurogenic atrophy in the peroneus brevis muscle and minor changes only in the superficial peroneal nerve. The mean age of onset was 12.06(range 3–25years), and progression was very slow.Inheritance appeared to be autosomal recessive. Depending on the clinical presentation, the patients were subdivided into three groups comprising(1) upper limb and sometimes bulbar amyotrophy with a bilateral pyramidal syndrome(17 patients: 11 familial and 6 isolated);(2) spastic paraplegia with peroneal muscular atrophy (14 patients: 11 familial and 3 isolated): and (3) a spastic pseudobulbar from (12 patients in a large kinship). These entities are discussed and compared with other cases reported in the literature.