Objective: This study tests the null hypothesis that the size of the pelvic opening spanned by the pelvic floor is the same in African American and European American women. Study Design: Forty African American female pelvises were age matched with 40 European American female pelvises from the Hamann-Todd collection at the Cleveland Museum of Natural History. The distances between the anchoring points of the pelvic floor to the bony pelvis (pubis anteriorly, ischial spines laterally, and inferior lateral angle of the sacrum posteriorly) were measured on each half of the pelvis. Measurements from left and right halves were averaged. The cross-sectional area of the pelvic floor was calculated from these dimensions. The bi-ischial line divided the total area into anterior and posterior pelvic floor areas. Analyses taking into account differences in stature by dividing individual dimensions by height were also performed. Group differences were compared with the Student t test and the Mann-Whitney rank sum test. Results: African American women had a 5.1% smaller pelvic floor area than European American women (889.6 cm² vs 937.0 cm², 5.1% P =.037). This was attributable to a 10.4% smaller posterior area (365.3 cm² vs 407.6 cm², 10.4% P =.016), whereas the anterior areas were similar (524.3 cm² vs 529.3 cm², P =.61). The following measured distances were smaller in African American women: ischial spine to inferior sacral angle (5.4 cm vs 5.9 cm, P =.016) and bi-ischial diameter (10.0 cm vs 10.6 cm, P =.004). These distances remained significant after height was controlled. Conclusions: In African American women, the posterior pelvic floor area is 10.4% smaller than in European American women, resulting in a 5.1% smaller total pelvic floor area. (Am J Obstet Gynecol 2002;187:111-5.)