Thermal sensitivity was studied in 280 type I diabetic patients and in 75 control subjects. Warm and cool thresholds, temperature sensitivity limen (difference between warm and cold thresholds, TSL), and hot and cold pain thresholds were quantitated on the skin of the index finger, hand, foot and medial calf. The diabetic group had mean values that were significantly different from controls in all variables except the pain thresholds in the upper extremity. TSL was the most sensitive parameter, being abnormal in 57, 63, 79 and 78% of patients in the four skin sites tested. Hot pain sensitivity was abnormal in 37, 21, 39 and 26% of patients in the same sites. Thermal sensitivity abnormalities were more frequently observed than abnormalities in motor and sensory nerve conduction studies. Thermal tests correlated with the duration of the diabetes, although there were abnormalities at all stages of the disease. The results show that diabetic neuropathy has a variable presentation in different types of nerve fibres.