Alteration of carpal morphometry after surgical release of the carpal tunnel was investigated in 22 patients with idiopathic carpal tunnel syndrome, using pre- and postoperative computed tomography. The morphometric evaluation showed an increase in carpal volume which was due to a palmar soft-tissue prolapse in 90% and to an increase of the carpal arch cross sectional area in about 10%. The carpal tunnel release caused a palmar opening of the canal most commonly at the pisiform bone. There was also a variation in form and an increase in volume of Guyon's canal. The results are discussed and related to the postoperative complaints.