Nerve conduction studies and measurements of thermal thresholds (hot and cold) were performed in 25 healthy control subjects and 24 patients with carpal tunnel syndrome: first, to compare the extent of abnormality demonstrable in the function of large and small afferent axons; second, to correlate these abnormalities with clinical severity; and third, to validate a modified form of palmar Stimulation as an additional neurophysiological test in suspected carpal tunnel syndrome. The routine nerve conduction studies compared the amplitudes and conduction velocities from digit II and digit V to wrist and for the digit4 potential between wrist and elbow. With palmar stimulation, the palmto-wrist segment was compared to the simultaneously evoked antidromic digital potential (palm-to-index). Thermal threshold testing involved determination of hot and cold thresholds for digit II and digit V using an automated forced-choice procedure. In the patients there was selective slowing of conduction across the palm-to-wrist segment with normal velocities distally and proximally, and there was clear evidence of abnormal small-fiber function, with higher thresholds for both hot and cold sensation. However, the thermal abnormalities also involved the ulnar territory, equally so for cold threshold but significantly less so for hot threshold. Thermal threshold testing confirmed the clinical impression that small-fiber dysfunction is important in carpal tunnel syndrome, but the abnormal findings for the ulnar-innervated digit V suggest that this test will not be routinely useful for the early detection of compression or entrapment neuropathies. 0 1994 John Wiley 8, Sons, Inc.