This study assessed the clinical use of three ultrasound measurements; median nerve cross-sectional area, median nerve flattening ratio and palmar displacement of the flexor retinaculum, for the diagnosis of carpal tunnel syndrome. The measurements were made in 20 carpal tunnel sufferers and 20 controls. The sensitivity, specificity and predictive values of each were calculated in various clinical settings. Values for each of the three variables were significantly different in the patient and control populations. The differences we recorded were smaller than those found in previous studies. The tests had a sensitivity of 72% and a specificity of 90%. Alterations in the morphology of the carpal tunnel in patients with carpal tunnel syndrome can be measured in the district general hospital setting. The measurements described maybe a useful non-invasive confirmatory test in patients in whom there is a strong clinical suspicion of carpal tunnel syndrome. However, they would be of no benefit in epidemiological surveys of populations with a low incidence of carpal tunnel syndrome.