The morphological correlation of the phenomenon of increased pressure in the carpal tunnel during wrist flexion and extension--as has been proved though measurements using wick-catheters--was studied in healthy subjects (n = 15) and symptomatic patients with carpal tunnel syndrome (n = 15). Our own measurements using magnetic resonance imaging (MRI) showed that there is a significant reproducible decrease in carpal tunnel diameter when the wrist is held in position of either flexion or extension. During flexion the diameter is decreased at the pisiformes and hamate level as well as it is lowered during extension at the pisiformes level. This might explain the rise in carpal tunnel pressure and thus the consecutive negative influence on the median nerve. Proximal swelling, distal flattening and increased signal intensity of the median nerve as well as the palmar bulging of the flexor retinaculum at the level of the hook of the hamate and at the level of the pisiformes were significantly higher in patients with carpal tunnel syndrome than in normal volunteers (from p < 0.05 to p < 0.001). In post-operative follow-up examinations of 13 patients with no clinic symptoms the distal flattening of the median nerve normalized in 94% within 3 months. The increased signal of the median nerve on T2-weighted images decreased postoperatively in 2/3 of the patients, whereas the motor latency of the median nerve recovered only in 39% of our patients who had 100% partial or complete clinical benefit. These findings imply that postoperative imaging may be helpful for evaluating the success or failure of surgical treatment.