We examined 66 hands referred with suspected carpal tunnel syndrome (CTS) using the second lumbrical-interosseous distal motor latency difference (2LI-DML) as well as standard tests: Forty-nine cases of CTS were diagnosed by the standard tests, 48 of whom had an abnormal median-ulnar palmar velocity comparison and 48 an abnormal 2LI-DML. The results of these 2 tests were closely correlated. The 2LI-DML supported the diagnosis of CTS in all cases except one, where the result was borderline. In one suspected case the 2LI-DML was the only abnormality. In 9 severe cases no median palmar responses could be obtained but an abnormal 2LI-DML was found. We conclude that the 2LI-DML is as sensitive as the palmar comparison and thus will support the diagnosis of CTS made by standard tests by providing an additional abnormality but that its routine use is unlikely to increase the diagnostic yield. Its value therefore may be in mild cases where the median-ulnar palmar comparison is normal or equivocal and in severe cases where standard test responses are unobtainable. It has also proved useful as a quick and simple screening test for CTS on the asymptomatic side.