Because digit 4 (D4) has dual innervation, median and ulnar sensory latencies can be determined over identical distances. To determine if D4 testing is more sensitive than other commonly used techniques to diagnose carpal tunnel syndrome (CTS), we examined 42 hands with clinical evidence of carpal tunnel syndrome and 43 control hands. D4 latency was significantly longer than controls more often than digit 2 (D2) in patients with CTS. Comparing median to ulnar latencies from D4 was the most sensitive method to make the diagnosis of CTS. In mild CTS, a characteristic double peak potential was seen in recordings from the median nerve after stimulating D4. This potential provided immediate visual confirmation of the diagnosis of CTS. Comparing D4 latency along median and ulnar nerves is useful for detecting mild CTS and should be used whenever there are clinical signs and symptoms of CTS but electrodiagnostic studies are normal or borderline.