For the diagnosis of carpal tunnel syndrome (CTS), a sensory conduction study of median nerve is the most sensitive parameter, by either antidromic or orthodromic recording. Many different sensory recordings have been developed to detect the mild or early cases of carpal tunnel syndrome. A comparison of the median and ulnar sensory responses using the 4th digit either orthodromically or antidromically has been one of the methods. However, a simultaneous comparison of both antidromic and orthodromic methods on the 4th digit has not been documented. For the comparison between the median and the ulnar sensory nerve conduction of the 4th digit recorded antidromically or orthodromically, conduction studies of the median and ulnar sensory nerves were performed using standard methods in normal populations as well as in patients with carpal tunnel syndrome. We studied 31 CTS patients (46 hands) with mean age of 54 years old (range, 25~70). Also, 51 subjects (102 hands) with mean age of 48 years old were studied as control. The difference of antidromic latencies between the median and the ulnar nerves was less than or equal to 0.4 msec in the control subjects and greater than or equal to 0.5 msec in the patients with carpal tunnel syndrome. The difference of orthodromic latencies was less than or equal to 0.5 msec in the control subjects and greater than or equal to 0.5 msec in the patients with carpal tunnel syndrome. By the antidromic and orthodromic methods, the mean difference between latencies of the median or ulnar nerve was not statistically significant. However the amplitude of median or ulnar nerve was 2 times larger by the antidromic method than by the orthodromic. We concluded that the latency difference of 0.5 msec or greater between the median and ulnar nerve sensory conductions from the 4th digit would be valuable for the diagnosis of CTS. The antidromic methods with larger amplitude may be more technically convenient to determine CTS than the orthodromic methods.