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While prophylactic phenylephrine infusions during cesarean delivery are often used to counteract the sympathectomy associated with spinal anesthesia, their use has been associated with decreased rostral spread of local anesthetic. The purpose of this study was to determine the median effective dose (ED50) and 95% effective dose (ED95) of intrathecal hyperbaric bupivacaine for cesarean delivery in the presence and absence of prophylactic phenylephrine infusion.One hundred healthy parturients undergoing elective cesarean delivery under combined spinal-epidural anesthesia were enrolled in this randomized, double-blind, dose-finding study to determine ED50 and ED95. Subjects were randomly assigned to receive prophylactic phenylephrine or control (saline) infusion with injection of 6, 7.5, 9, 10.5, or 12 mg intrathecal hyperbaric bupivacaine. An effective dose was defined as achieving a T5 sensory level within 10 minutes of intrathecal drug administration without the need for epidural supplementation.The ED50 of intrathecal hyperbaric bupivacaine was 10.0 mg (95% confidence interval [CI], 5.9–11.4 mg) with prophylactic phenylephrine infusion and 7.9 mg (95% CI, 2.3–9.4 mg) in the control group. The ED95 of intrathecal hyperbaric bupivacaine was 14.1 mg (95% CI, 12.3–37.6 mg) with prophylactic phenylephrine infusion and 11.7 mg (95% CI, 9.9–22.8 mg) in the control group.The administration of prophylactic phenylephrine infusion results in higher intrathecal hyperbaric bupivacaine dosing requirements in parturients undergoing cesarean delivery.This study was registered at the Chinese Clinical Trial Registry, identifier ChiCTR-TRC-16008938.