Computed tomography was used to measure the cross-sectional area of the carpal canals in normal controls of both sexes and in women with idiopathic carpal tunnel syndrome. The women controls had significantly smaller carpal canals than the men controls both proximally and distally. In the patients both the proximal and distal cross-sectional areas were significantly reduced compared with the women controls. The measurements showed that carpal canal stenosis is associated with idiopathic carpal tunnel syndrome, narrowing of the canal is bilateral in patients who have unilateral symptoms, and narrowing is greater in the proximal carpal canal. There was no correlation between age and the size of the canal. The difference in the size of the carpal canal between normal men and women might explain the tendency of women to develop carpal tunnel syndrome. The lack of correlation between age and the size of the canal suggests that stenosis of the carpal canal is inherited rather than acquired. Symptoms arise only later in life, when degenerative changes in the content or the walls of the carpal canal compete with the median nerve for space and its function becomes impaired by compression.