Autonomic neuropathy, especially sympathicopathy, a life-threatening complication of diabetes mellitus, has been difficult to evaluate, and remains undetermined. We studied the thermographic patterns of 62 patients (50-69 years old), revealing the vasodilated activities of one leg by immersing the other leg in a warm water bath, for the detection of sympathetic neuropathy, using Thermoviewer MDJTG-MD. The normal pattern shows an increase in skin temperature, while the flat pattern shows no rise or even a paradoxical decrease. The latter had a significantly longer duration of diabetes and poor blood sugar control. The thermographic pattern is closely related to microangiopathy, R-R interval variation and motor nerve conduction velocity. However, R-R interval variation is often abnormal in patients with normal thermographic patterns, showing vasosympathetic abnormalities appearing far later in the development of diabetic neuropathy. The flat pattern develops slowly after at least several years of poor blood sugar control. Change in the pattern is also gradual and slow. The reproducibility of the pattern is excellent and requires relatively simple and noninvasive techniques. Thermography is one of the most reliable, reproducible and noninvasive indexes for finding and following diabetic sympathetic abnormalities.