Asymptomatic ulnar neuropathy in carpal tunnel syndrome. Quantitative assessment of the vibration threshold of the second and fifth fingertips at 125Hz was performed on 28 affected limbs of 17 patients with carpal tunnel syndrome (CTS) together with a median and ulnar sensory nerve conduction velocity (SNCV) study. Twenty-six limbs of 26 age-matched healthy subjects were used as controls. Both the vibration threshold elevation of the second finger and the delay of median SNCV were significant in CTS patients as compared with controls (p less than .001). Although the ulnar SNCVs of all affected limbs were within the normal range, ten affected limbs (36%) were found to have a concomitant vibration threshold elevation of the fifth finger, and eight of those limbs were found to be exposed to chronic occupational mechanical stimulation at the wrist. These findings appear to support the presence of subclinical traumatic damage to the ulnar nerve at the wrist. In summary, digital vibration measurement can elucidate subclinical ulnar neuropathy in CTS which cannot be detected by SNCV studies. 