Sensory conduction velocity of the median nerve, motor conduction velocity of both median and tibial nerves, and corresponding distal laterncies are sufficient parameters to establish the diagnosis of polyneuropathy almost with certainty. Considering these six parameters yielded in detection of peripheral nerve dysfunction in 22% of diabetic patients who were free from clinical signs of polyneuropathy. Electroneurographical findings in 340 out of 677 patients with diabetes mellitus were interpreted as evidence of segmental demyelination. Within this group there was the majority of patients with clinical signs of polyneuropathy and with subclinical signs of peripheral nerve dysfunction. There existed a positive correlation between signs of nerve dysfunction with angiopathy, age and duration of the disease. A second group consisting of 243 diabetics with signs of incipient segmental demyelination with or without signs of axonaal degeneration mainly included juvenile patients with a short duration of the disease and with a low frequency of angiopathy.