The purpose of the present study was to examine the value of measuring sensory conduction along the median and ulnar nerves of the fourth finger in the diagnosis of a carpal tunnel syndrome (CTS). In 23 controls, sensory conductions along median and ulnar nerves were identical. In 28 of 38 patients with CTS, stimulation of the ring finger revealed a reduced conduction velocity along sensory median nerve fibres in contrast to normal conduction along ulnar sensory nerve fibres. In 5 patients, a sensory action potential was absent over the median nerve and in another 5 sensory conduction was normal along both nerves. We conclude that testing of sensory conduction along the ring finger is useful in about 74% of patients with CTS, while in the remaining 26% other fingers must be examined to establish the diagnosis.