We studied pressure in the carpal tunnel in patients with carpal tunnel syndrome and in normal control subjects, using a slit catheter and recording in the neutral position, 90 degrees dorsiflexion, and 90 degrees palmarflexion of the wrist. For each position the mean pressure in the patients was very significantly higher than in the controls, the highest pressure being in 90 degrees dorsiflexion, and the lowest in the neutral position. Using an upper limit of normal pressure of 5.5 mmHg in the neutral position gave a diagnostic sensitivity of 78.7%, a specificity of 78.1%, an accuracy of 78.5%, and a positive predictive value of 87.3%.