AIM: The benefits of maintaining the pulley function of the flexor retinaculum in carpal tunnel release by lengthening or reconstructing it have been described. Quantitative MR imaging was used to investigate the morphological changes after open carpal tunnel release by such a retinaculum lengthening technique. METHOD: Ten patients had bilateral carpal tunnel MRI pre- and postoperatively. The MRI examinations were performed with a 1.5 Tesla imaging system and wrist coils. Carpal tunnel volume, carpal arch width, median nerve position and flexor tendon position in relation to the hamate-trapezial axis were recorded . RESULTS: Like other methods of carpal tunnel release with complete division of the flexor retinaculum, the retinaculum lengthening technique showed a significant postoperative increase of carpal tunnel volume. Carpal arch width increased only slightly. There was a significant palmar displacement of the median nerve but not of the flexor tendons. CONCLUSION: The findings support the hypothesis that maintenance of the pulley function of the retinaculum may lead to an early postoperative recovery of grip strength. In spite of some difficulties in application quantitative MR imaging may be a useful tool in evaluating the carpal tunnel morphology.