Abstract: Objective To determine the 50% effective dose（ED ₅₀ ）of intrathecal isobaric bupivacaine in combined spinal-epidural anaesthesia with epidural volume extension for caesarean surgery. Methods Eighty-four women undergoing elective caesarean sections were randomly assigned to seven groups, receiving intrathecally isobaric bupivacaine in a dosage of 6, 7, 8, 9, 10, 11 or 12 mg (n=12 per group), respectively. Ten mL normal saline was injected through an epidural catheter at a rate of 0.5 mL/s. Successful induction was defined as Hollmen pinprick block grade 2 at a height equal to or higher than T6. Success (surgery) was defined as success (induction) plus no requirement for epidural supplementation throughout surgery. The ED ₅₀ values were determined using a logistic regression model. Results The ED ₅₀ for success (induction) and success (surgery) were 8.121 mg ( 95% confidence interval: 7.421-8.821) and 9.012 mg (95% confidence interval:7.747-10.280), respectively. With increase in amount of intrathecal bupivacaine, time to success (induction) was shortened ( P<0.05), resulting in increased successful numbers for both induction and surgery. No differences were found in the use of phenylephrine, haemodynamic stability and the occurrence of nausea and vomiting among groups. Conclusion When combined with epidural volume extension, intrathecal bupivacaine has an ED ₅₀ of 8.121 mg and 9.012 mg for success (induction) and success (surgery), respectively, in women undergoing caesarean sections.