We investigated the reliability of some US signs in the diagnosis of the carpal tunnel syndrome. We carried out a single-blind study with 13-MHz high resolution probes and electromyography on 132 patients with clinical evidence of the carpal tunnel syndrome; a control group of 20 asymptomatic patients was also submitted to US. Eighty-six of 107 patients with US signs of the carpal tunnel syndrome were then submitted to surgical decompression (resection of the transverse carpal ligament), while the extant 21 patients underwent conservative treatment and clinical follow-up. To diagnose the carpal tunnel syndrome, we considered the following US patterns: median nerve changes (swelling before its entrance into the carpal tunnel and flattening in the tunnel itself), palmar bowing of the flexor retinaculum, thickening of the transverse carpal ligament and increased depth of the carpal tunnel, as measured from the apex of the transverse carpal ligament convexity to the underlying carpal bone. Median nerve changes were unreliable signs and were missing in many cases: only 45 of 107 patients exhibited median nerve swelling before and/or its flattening in the carpal tunnel (42%). Such indirect signs as the thickening of the transverse carpal ligament in chronic cases were demonstrated in 94 of 107 patients with the carpal tunnel syndrome (88%) and canal deepening in all unilateral carpal tunnel syndromes was shown in 92 of 107 patients (87%); both these signs proved to be much more reliable. The palmar bowing of the flexor retinaculum was also difficult to demonstrate in surgical patients or in those with connective tissue fibrosis within the tunnel: this sign was demonstrated in 80 of 107 patients with the carpal tunnel syndrome confirmed with electromyography (75%). Tanzer and Rietze reported median nerve changes observed at surgery in 43% and 66% of their patients, respectively. Recent MR findings in asymptomatic wrists have demonstrated that the normal median nerve has an elliptical shape inside the carpal tunnel. To conclude, high resolution US exhibited 96% sensitivity, 95% specificity and 93% diagnostic accuracy and proved to play a major role in the diagnosis of the carpal tunnel syndrome.