In 1986 the fetal-pelvic index was introduced as an accurate method of prospectively identifying the presence or absence of fetal-pelvic disproportion. The concept of the fetal-pelvic index is one in which the fetal head and abdominal circumferences (ultrasonographic mensuration) are compared with the respective maternal pelvic inlet and midpelvic circumferences (x-ray pelvimetry). The purpose of this study is to evaluate the efficacy of this index as a predictor of fetal-pelvic disproportion in gravid women attempting vaginal birth after previous cesarean delivery. Findings of the fetal-pelvic index are compared with those of two other means of identifying fetal-pelvic disproportion (Colcher-Sussman x-ray pelvimetry and ultrasonographically derived estimated fetal weight of greater than or equal to 4000 gm). Of the 65 prospective study participants, 18 (28%) failed to progress in labor, and a repeat cesarean delivery was required. Forty-seven of the 52 patients with a negative fetal-pelvic index value were delivered vaginally (negative predictability = 0.90). Of the five with false-negative fetal-pelvic index values, four fetuses persisted in an occipitoposterior position and failed to progress in labor. All 13 patients with a positive fetal-pelvic index value failed to progress in labor and required a cesarean delivery (positive predictability = 1.00). In contrast, when used alone, neither x-ray pelvimetry nor ultrasonography-determined estimated fetal weight of greater than or equal to 4000 gm provided accurate identification of fetal-pelvic disproportion.