Objectives: This study was undertaken to assess the depth of the pouch of Douglas among nulliparous women and to compare it with those among parous women and patients with genital prolapse. Study Design: The depth of the pouch of Douglas was measured in 22 nulliparous women and 28 parous women without genital prolapse and in 18 patients with prolapse of the posterior vaginal wall. Results: The pouch of Douglas extended from 11% to 89% of the total vaginal length among nulliparous and parous women, without significant difference between these groups (mean, 49% and 45%, respectively). It was significantly deeper in patients with prolapse, ranging from 25% to 96% (mean, 72%). Conclusion: There is an extensive variation in the depth of the pouch of Douglas. A deep pouch of Douglas is frequently present in young nulliparous women and may increase the risk for development of a posterior vaginal wall prolapse. (Am J Obstet Gynecol 2000;182:540-4.)