BACKGROUND Intrathecal anesthesia is commonly used for cesarean section. Bupivacaine and ropivacaine have all been used as intrathecal drugs. The minimum effective local anesthetic dose (MLAD) of intrathecal ropivacaine for non-obstetric patients has been reported. However, few data are available on the MLAD of hyperbaric ropivacaine for obstetric patients and the relative potency to bupivacaine has not been fully determined. In this study, we sought to determine the MLAD of intrathecal ropivacaine and bupivacaine for elective cesarean section and to define their relative potency ratio. METHODS We enrolled forty parturients undergoing elective cesarean section under combined spinal-epidural anesthesia and randomized them to one of two groups to receive intrathecal 0.5% hyperbaric ropivacaine or bupivacaine. The initial dose was 10 mg, and was increased in increments of 1 mg, using the technique of up-down sequential allocation. Efficacy was accepted if adequate sensory dermatomal anesthesia to pin prick to T7 or higher was attained within 20 minutes after intrathecal injection, and required no supplementary epidural injection for procedure until at least 50 minutes after the intrathecal injection. RESULTS The intrathecal MLAD was 9.45 mg (95%confidence interval (CI), 8.45 - 10.56 mg) for ropivacaine and 7.53 mg (95%CI, 7.00 - 8.10 mg) for bupivacaine. The relative potency ratio was 0.80 (95%CI, 0.74 - 0.85) for ropivacaine/bupivacaine when given intrathecally in cesarean section. CONCLUSIONS Ropivacaine is 20% less potent than bupivacaine during intrathecal anesthesia for cesarean delivery.