Objective: The purpose of this study was to compare the smooth muscle content of the anterior vaginal wall in normal women and women with pelvic organ prolapse. Study Design: Specimens were taken from the apex of the anterior vaginal cuff after abdominal hysterectomy from 28 women with pelvic organ prolapse and 12 control subjects. Smooth muscle cells of the anterior vaginal wall were identified by immunohistochemistry with antibodies to smooth muscle α-actin. Morphometric analysis was used to determine the fractional area of nonvascular smooth muscle in the muscularis in histologic cross-sections of the anterior vaginal wall. Results: The fractional area of nonvascular vaginal smooth muscle in the muscularis of women with prolapse was significantly decreased compared with that of control subjects. This decreased fraction of smooth muscle in the anterior vaginal wall was not related to age, race, or stage of prolapse. In women with prolapse, vaginal smooth muscle content was most diminished in specimens from postmenopausal women with no estrogen replacement. The fractional area of muscularis smooth muscle was also decreased significantly in premenopausal women with prolapse. Conclusion: The fraction of smooth muscle in the muscularis of the anterior vaginal wall is significantly decreased in women with pelvic organ prolapse compared with normal control subjects. (Am J Obstet Gynecol 2002;187:56-63.)