The effect of temperature on normal nerves is well known, i.e., an increase in conduction velocity and a decrease in amplitude with an elevation in temperature. There are few reports examining the effect of temperature on abnormal nerves, e.g., in demyelination. To study the effect of increased temperature on demyelinating nerves in entrapment syndromes, the authors investigated 55 median and 48 ulnar nerves of 48 patients with carpal tunnel syndrome, and 48 median and 48 ulnar nerves of 26 healthy subjects. All measurements were obtained at 32 degrees C and 37 degrees C. Mean reductions in median sensory amplitude occurring with heating were significantly greater in the patient group than in the control group (P = 0.000). For median sensory response amplitude, the mean decrease was 32.1% in patients with carpal tunnel syndrome and 10.7% in the control subjects. The difference between median and ulnar nerves in the latency was significantly decreased (P = 0.027) after the nerves had heated to 37 degrees C. It is concluded that the elevation in temperature leads to conduction block in demyelinated sensory nerves, and that temperature provocation may be useful in the diagnosis of nerve disorders. The effect may be different in axonal and demyelinating disorders.