Clinical and neurophysiological studies were undertaken, with particular reference to the arms, in 39 patients with diabetic neuropathy. The effects of an aldose reductase inhibitor, sorbinil, on neuropathy in these patients were studied in a 12 month double blind placebo controlled trial. Neurophysiological measurements, particularly of sensory amplitude, were considerably more sensitive than measurements of temperature and vibration sensation and remain of fundamental importance in measuring diabetic neuropathy at an early and potentially reversible stage. There was no significantly beneficial effect of sorbinil on clinical or neurophysiological measurements of nerve function in patients with established diabetic neuropathy. These results indicate that neurophysiological techniques are necessary, in conjunction with clinical measurements, for the assessment of 'early' diabetic neuropathy and that aldose reductase inhibitors are not effective in the treatment of established diabetic neuropathy.