Continuous-wave Doppler studies of the umbilical arteries were performed on 100 women with placenta previa prior to delivery in the third trimester and 100 control subjects matched for gestational age and with normally implanted placentas. The ratio of the maximum (systole [S]) and minimum (diastole [D]) velocities appeared to reflect downstream vascular resistance. An S:D ratio of greater than or equal to 3.0 indicated elevation. There was a statistically higher S:D ratio in the placenta previa group as compared to the control group, and that difference persisted even when growth-retarded fetuses were excluded from both groups. Placenta previa patients with S:D ratios greater than or equal to 3.0 were associated with a statistically higher incidence of adverse pregnancy outcomes. The sensitivity, specificity, and positive and negative predictive value of an elevated S:D ratio in predicting adverse pregnancy outcomes were 90.0%, 93.8%, 62.0% and 98.5%, respectively. Our data suggest that (1) placenta previa patients are at increased risk of intrauterine growth retardation and adverse pregnancy outcomes, (2) placenta previa patients with normal S:D ratios are at no greater risk of adverse outcomes than are controls, (3) when the S:D ratio is elevated in placenta previa patients, it is more likely to be associated with adverse pregnancy outcomes, and (4) placenta previa patients without growth failure have a higher S:D ratio than do those with normally implanted placentas. Umbilical artery velocimetry appears to be a useful adjunct to ultrasound in the surveillance of pregnancies complicated by placenta previa.