Objective To study the characteristics of nociceptive system in subacute combined degeneration (SCD) by contact heat evoked potentials (CHEPs),and evaluate the CHEPs in the electrophysiological diagnosis of SCD.Methods Twenty definite SCD patients and 34 sex-,height-and age-matched healthy controls were included in this study.Thermal stimuli was applied at 51 ℃ intensity level at four sites:C_7,volar surface of the forearm,the skin of leg at 5 cm proximal to the medial malleolus and lumbar part The CHEPs were recorded from Cz.The latency and amplitude of the evoked potentials were recorded.The CV of Aδ fibers of peripheral nerves and N-wave latencies and N-P amplitudes of the central nerves were analyzed.The somatosensory evoked potential (SEP) and sensory conduction dsvelocity (SCV) of the limbs were also evaluated.Results (1) CHEPs were elicited reliably and stably in all controls.(2) The visual analogue scale (VAS) for pain perception was higher in controls (upper limbs:7.6 and lower limbs:6.9) as compared to SCD complication with hypoesthesia (upper limbs:6.4 and lower limbs:5.7,Z = 3.478 and 3.909,both P < 0.01).(3) The conduction velocity of Aδ fibers in upper and lower limbs in the patients was significantly reduced compared to the controls ((9.77 ± 4.28) m/s vs (12.87±1.40) m/s;(8.19±0.91) m/s vs (12.87±1.40) m/s,respectively,t = 3.142 and 14.864,both P<0.01).Thirteen out of 20 patients (65%) had slow CV of Aδ fibers,however only 4 out of 20 patients (20%) had slow SCV of median nerve;19 out of 20 patients (95%) had slow CV of Aδ fibers,however only 12 out of 20 patients (60%) had slow SCV of tibial nerve.SCD have higher abnormality rates in CHEPs than SCV (P<0.05).(4) In the lower limbs,16 out of 20 patients (80%) had markedly prolonged CHEPs while 18 out of 20 patients (90%) had markedly prolonged SEP.Conclusions The study suggests that CHEPs can quantitatively detect the dysfunction of nocieptive system in SCD,and is more sensitive than SCV.CHEPs may be used as a convenient and objective measure for the diagnosis of SCD in electrophysioiogy.