The purpose of this study was to assess placental vascular resistance as measured by peak systolic to end-diastolic ratios (S/D) of the umbilical artery waveforms in patients with late decelerations. Fifty patients with persistent late decelerations and 50 gestational age-matched controls with reactive tracings underwent umbilical velocimetry with measurement of S/D ratios. The mean (+/- SD) S/D ratio in the late-deceleration group (4.32 +/- 1.1) was significantly higher than that of the control group (2.32 +/- 0.21) (P less than .01). Patients with late decelerations had a significantly higher incidence of adverse pregnancy outcome, as judged by the incidence of small for gestational age infants, meconium, cesarean section for fetal distress, low Apgar scores, and neonatal intensive care unit admissions, compared with the control group. However, there was no difference in the incidence of adverse pregnancy outcome in the late-deceleration group with normal S/D ratios compared with controls. The sensitivity, specificity, and positive and negative predictive values of S/D ratios in the late-deceleration group for predicting adverse pregnancy outcome were 85.7, 100, 100, and 89.6%, respectively. These data suggest that umbilical velocimetry might be able to discriminate late decelerations that constitute a false-positive test. Incorporation of umbilical velocimetry into antepartum and intrapartum fetal surveillance schemes may therefore be helpful.