The uterosacral ligament (USL) is part of the pelvic floor connecting the uterus to the dorsal body wall and stabilizing it. In samples obtained from hysterectomy patients, the smooth muscle component of the cervical third of the USL was studied by smooth muscle actin (SMA) immunohistochemistry as indicator for structural abnormalities and by morphometric determination of nuclear size as measure for the functional state. From a total of 40 patients, 18 were matched according to age, parity, and premenopausal status. Immunohistochemistry showed a granular patchy distribution of SMA in the pelvic organ prolapse (POP) group. In patients with POP, the size of smooth muscle cell nuclei was significantly reduced in comparison to patients without POP (25.45+/-1.92 microm(2) vs 28.87+/-2.92 microm(2), p<0.05). We conclude that the smooth muscle component of the USL is impaired in POP patients, indicating a possible role of smooth muscle in the pathogenesis of POP.