Increased prevalence of impaired glucose tolerance (IGT) has been recently detected in patients with painful sensory neuropathy. To determine whether nerve abnormalities are present in IGT we investigated IGT subjects without clinical neuropathy. Nerve conduction studies (NCS) were performed in 12 subjects with IGT without symptoms and signs of neuropathy. The results were compared with those obtained from 12 patients with type 2 diabetes (DM) without clinical neuropathy and 12 healthy controls. Sensory NCS of the sural nerve were performed on different segments, the distal-leg (10 cm proximal to the lateral malleolus) and the proximal-leg segment (10 cm more proximal). The distal conduction velocity of the sural nerve was increased in IGT subjects, compared both to healthy controls and DM patients. No difference was found among the groups with respect to the sensory conduction velocity of the sural nerve fibers in the proximal-leg segment. A reduction of both distal and proximal amplitudes of the sural nerve action potentials was detected in DM patients compared with IGT subjects and controls. The abnormal conduction velocity in the distal segment of the sural nerve, observed in IGT subjects without clinical neuropathy, suggests that the myelin dysfunction of the distal sensory fibers represents the earliest detectable nerve response to the hyperglycemia. The reduced amplitude of the sural nerve action potential in asymptomatic patients with DM arises from the axonal degeneration and represents a more advanced stage of nerve disease.