A study of interstitial pressures within the carpal tunnel using a slit catheter found that, in some patients, an initial rise in pressure is recorded when the wrist is passively extended a this continues to rise to a plateau if the position is maintained. A rise above a critical pressure brought about by congestion would explain the clinical picture of predominantly nocturnal symptoms and no electro-physiological evidence with the wrist in a neutral (resting) position. The results also bring into doubt published results of the pressure within the carpal tunnel wit the wrist flexed or extended, since the pressure can be changed at will, depending on the flexion or extension force used.