The relationship of plantar skin temperature to diabetic neuropathy was studied using clinical, nerve conduction and autonomic evaluations. The sympathetic skin response (SSR) was found present in both feet of 25 control subjects and 29 (out of 69) diabetic patients (SSR+ group). For those diabetic patients absent with the SSR in both feet, 18 patients (at-risk group) had preulcerative skin lesions (dry and fissured skin) and 22 did not (SSR- group). The at-risk group showed significantly higher mean foot temperature (30.2+/-1.3 degrees Celsius) than the SSR- (27.9+/-1.7 degrees Celsius), the SSR+ (27.1+/-2.0 degrees Celsius) and the control group (26.8+/-1.8 degrees Celsius). The SSR- group had smaller temperature differences (7.2+/-1.7 degrees Celsius versus 8.6+/-1.6 degrees Celsius, p<0.05) and smaller normalized temperature (referencing to the forehead temperature) (0.19 versus 0.24, p<0.01) than the SSR+ group. Although the three diabetic groups had no significant differences in clinical and cardiovascular abnormalities, the at-risk group showed more nerve conduction abnormalities than the SSR- and SSR+ groups (55% versus 23% and 14%, p<0.02). This study indicated that the thermoregulatory sweating abnormality signified early sympathetic damage in diabetic feet. Assessing skin conditions and sudomotor activities should help identify small fiber neuropathy in diabetic patients with at-risk feet conditions.