Our objective was to compare the collagen and muscle content of the round ligament of uteri in women with pelvic organ prolapse. We evaluated the tissue samples obtained from the round ligaments of 22 patients with uterine prolapse who underwent vaginal hysterectomy (group A, study) and from 26 patients with no pelvic relaxation in whom total abdominal hysterectomy was performed for benign reasons (group B, controls). Morphometric analysis was performed on histologic cross-sections of the round ligament. Sections from each sample were stained with hematoxylin and eosin and Masson's trichrome methods. A computer system was used for morphometric measurements. We used independent samples t-test or Mann-Whitney U test to investigate the difference between the two groups. It was found that the smooth muscle fraction of the round ligament in women with uterine prolapse was significantly decreased compared with that of healthy control subjects and concluded that decreased smooth muscle content may be an important pathogenetic factor in uterine prolapse.