Because the fourth digit (D4) has a dual innervation, median and ulnar D4 sensory conduction velocity (SCV) comparison may be useful in diagnosing the carpal tunnel syndrome (CTS). We studied 50 control hands and 41 hands with recent onset symptoms and signs of CTS but normal median distal motor latency and normal SCV from the second digit (D2). In CTS, D4 SCV was significantly slower than D2 SCV and D4 median and ulnar sensory conduction difference was abnormal in 38 hands (92%). In 36 CTS hands (87%), but in no control hand, a double peak potential could be recorded over the median after D4 stimulation providing an immediate visual confirmation of the diagnosis of CTS. Comparing median and ulnar D4 SCV is a very sensitive method to detect early or mild CTS and should be used whenever conventional electrodiagnostic studies are normal or borderline.