Diabetic peripheral neuropathy affects both large myelinated and small unmyelinated nerve fibres. It has been proposed that the small unmyelinated fibres, responsible for pain and temperature sense, and autonomic function, are involved early, particularly in subjects with painful symptoms, and may be important in foot ulceration. The sympathetic skin response has been used to investigate the function of small unmyelinated sympathetic fibres in the limbs of diabetic subjects. Changes in skin resistance at the fingers and toes have been measured simultaneously after a sound stimulus. These procedures were controlled using a microcomputer. Data collected from 55 diabetic subjects, randomly selected from the diabetic clinic, have been compared with results from conventional tests of large motor and sensory fibres and autonomic function. The ratio of the change in skin resistance for toes to fingers correlated with sural and posterior tibial nerve conduction velocity (correlation coefficients 0.54 and 0.42, p less than 0.001 and p less than 0.01, respectively), with the expired to inspired ratio (correlation coefficient 0.51, p less than 0.01), and inversely with vibration perception threshold in the feet (correlation coefficient 0.50, p less than 0.001). Correlation with the dark adapted pupil diameter, however, only just achieved statistical significance (correlation coefficient 0.27, p = 0.043). We propose that this simple test may elucidate the role of the peripheral autonomic system in diabetic neuropathy.