Objective:To determine the potency of epidural ropivacaine in inhibiting breakthrough pain in primiparae undergoing labor analgesia with programmed intermittent epidural bolus (PIEB).Methods:American Society of Anesthesiologists Physical Status classification Ⅰ or Ⅱ primiparae of full-termpregnancy, with a singleton fetus in vertex presentation, aged ≥18 yr, with body mass index < 30 kg/m 2, presenting with breakthrough pain during labor analgesia with PIEB, were enrolled in this study. Ropivacaine 10 ml was epidurally administered, and the concentration was determined by up-and-down sequential allocation. The initial concentration was set at 0.15% in the first patient in each group. Each time the concentration increased/decreased in the next patient depending on whether the patients showed breakthrough pain relief, and the ratio between the two successive doses was 0.9. The criterion of breakthrough pain relief was defined as numerical rating scale score < 4 points within 30 min after epidural injection of ropivacaine. The median effective concentration (EC 50) and 95% confidence interval of ropivacaine in inhibiting breakthrough pain were calculated by Dixon-Massey′s method. Results:Twenty-six patients were finally included in this study.The EC 50 (95% confidence interval)of ropivacaine in inhibiting breakthrough pain was 0.102% (0.088%-0.117%). Conclusions:The EC 50(95% confidence interval) of epidurally administered ropivacaine 10 ml is 0.102%(0.088%-0.117%) when used for inhibiting breakthrough pain during labor analgesia with PIEB in primiparae.