We report electrophysiological findings of conduction along peripheral sensory fibers in 64 patients with amyotrophic lateral sclerosis. Distribution of the values of action potential amplitudes and conduction velocities of peripheral afferent fibers were significantly lower than in normal age-matched controls. Sensory action potential amplitudes (SAPas) were more affected than sensory conduction velocities (SCVs). When single patients were considered, SAPas were slightly but significantly reduced in 22% of the cases (median nerve 17%, ulnar nerve 11%, and sural nerve 22%). A parallel decrease in SCVs and MCVs in 14 patients in whom the study was repeated over a period of time was also found. All these electrophysiological findings are due to progressive neuronopathy of peripheral sensory fibers. A pathogenetic mechanism is proposed.