Six patients presented with amyotrophy confined to a single lower limb and characterized by insidious onset, slow progression and later stabilization. Wasting was out of proportion with disability and there were no sensory, pyramidal tract or bulbar signs. All cases were sporadic, and there was no history of poliomyelitis. CK, anti-ganglioside antibodies, motor and sensory conductions were normal. Quantitative EMG and muscle biopsy revealed neurogenic features also in clinically unaffected limbs. Muscular CT showed selective or predominant, asymmetrical involvement of posterior leg muscles and caput longus of biceps femoris. Monomelic amyotrophy of lower limb is a clinically localized variant of spinal muscular atrophy with a particularly benign course. Although in the early stage there are no clinical or laboratory findings which allow differential diagnosis with other motor neuron diseases, the history of an amyotrophy clinically localized for more than 3 years to a lower single limb and the characteristic muscular CT pattern suggest the diagnosis since the first observation and indicate a favorable prognosis.