Higher-dose oxytocin is more effective than lower-dose regimens to prevent postpartum hemorrhage after cesarean delivery. We compared two higher-dose regimens (80 units and 40 units) to our routine regimen (10 units) among women who delivered vaginally. In a double-masked randomized trial, oxytocin (80 units, 40 units, or 10 units) was administered in 500 mL over 1 hour after placental delivery. The primary outcome was a composite of any treatment of uterine atony or hemorrhage. Prespecified secondary outcomes included outcomes in the primary composite and a decline of 6% or more in hematocrit. A sample size of 600 per group (N=1,800) was planned to compare each of the 80-unit and 40-unit groups to the 10-unit group. At planned interim review (n=1,201), enrollment in the 40-unit group was stopped for futility and enrollment continued in the other groups. Of 2,869 women, 1,798 were randomized as follows: 658 to 80 units; 481 to 40 units; and 659 to 10 units. Most characteristics were similar across groups. The risk of the primary outcome in the 80-unit group (6%; relative risk [RR] 0.93, 95% confidence interval [CI] 0.62-1.40) or the 40-unit group (6%; RR 0.94, 95% CI 0.61-1.47) was not different compared with the 10-unit group (7%). Treatment with additional oxytocin after the first hour was less frequent with 80 units compared with 10 units (RR 0.41, 95% CI 0.19-0.88), as was a 6% or more decline in hematocrit (RR 0.83, 95% CI 0.69-0.99); both outcomes declined with increasing oxytocin dose. Outcomes were similar between the 40-unit and 10-unit groups. Compared with 10 units, 80 units or 40 units of prophylactic oxytocin did not reduce overall postpartum hemorrhage treatment when administered in 500 mL over 1 hour for vaginal delivery. Eighty units decreased the need for additional oxytocin and the risk of a decline in hematocrit of 6% or more. ClinicalTrials.gov, , NCT00790062. I. © 2012 Lippincott Williams & Wilkins, Inc.