Microneurographic recordings of naturally occurring nerve activity in the median or peroneal nerve were made in 25 patients with diabetes mellitus, 17 of whom had signs of polyneuropathy. In patients without polyneuropathy, the electrical findings did not differ from those in healthy subjects. In patients with polyneuropathy, sensory afferent impulses were always normal qualitatively, whereas muscular afferent activity was weak or entirely absent in some patients. Sympathetic activity, if found, showed normal characteristics in muscle and skin fascicles, except that it was difficult to obtain a good signal-to-noise ratio. In 16 of 25 recordings with the electrode positioned intraneurally, sympathetic activity could not be detected in patients with polyneuropathy. The failures correlated with impaired skin sympathetic effector organ responses and reduction of motor nerve conduction velocity. The results suggest that impairment of sympathetic outflow occurs frequently in diabetic polyneuropathy and that sympathetic involvement occurs earlier than in many other types of polyneuropathy.