BackgroundThe minimum local analgesic concentration (MLAC) has been defined as the median effective local analgesic concentration in a 20‐ml volume for epidural analgesia in the first stage of labor. The aim of this study was to assess the relative analgesic potencies of epidural bupivacaine and ropivacaine by determining their respective minimum local analgesic concentrations.MethodsSeventy‐three parturients at <or= to 7 cm cervical dilation who requested epidural analgesia were allocated to one of two groups in this double‐blinded, randomized, prospective study. After a lumbar epidural catheter was placed, 20 ml of the test solution was given, either ropivacaine (n = 34) or bupivacaine (n = 39). The concentration of local anesthetic was determined by the response of the previous patient in that group to a higher or lower concentration using up‐down sequential allocation. Analgesic efficacy was assessed using 100‐mm visual analog pain scores with <or= to 10 mm within 30 min defined as effective. An effective result directed a 0.01% wt/vol decrement for the next patient. An ineffective result directed a 0.01% wt/vol increment.ResultsThe minimum local analgesic concentration of ropivacaine was 0.111% wt/vol (95% confidence interval, 0.100‐0.122), and the minimum local analgesic concentration of bupivacaine was 0.067% wt/vol (95% confidence interval, 0.052‐0.082). Ropivacaine was significantly less potent than bupivacaine, with a potency ratio of 0.6 (95% confidence interval, 0.49‐0.74). No difference in motor effects was observed.ConclusionRopivacaine was significantly less potent than bupivacaine for epidural analgesia in the first stage of labor.