Objective To determine the optimum dose of bupivacaine for spinal anesthesia with epidural volume extension (EVE) in patients undergoing cesarean delivery.Methods A total of 100 parturients,weighing 55-100 kg,with body height of 155-170 cm,of American Society of Anesthesiologists physical status Ⅰ or Ⅱ,scheduled for elective cesarean delivery,were included and randomized to receive bupivacaine at 5 different doses (A,B,C,D,and E groups) using a random number table,with 20 patients in each group.In A,B,C,D,and E groups,bupivacaine 6.75,7.50,8.25,9.00 and 9.75 mg were injected into the subarachnoid space,respectively,and the volume was all 1.5 ml.After successful spinal anesthesia,an epidural catheter was placed,and normal saline 10 ml was given through the catheter to perform EVE.A successful spinal block was defined as attainment of sensory block of at least T5 within 15 min after EVE along without requirement for additional local anesthetics within 60 min after EVE.The rate of successful spinal anesthesia,time to reach sensory block of T5,the maximum level of sensory block,and the first requirement for local anesthetics were recorded.The occurrence of hypotension,tachycardia,bradycardia,nausea,vomiting,chest distress,dyspnea,shivering,and referred pain and consumption of metaraminol were recorded during spinal anesthesia.Apgar scores at 1 and 5 min after birth were recorded.Results With the increasing doses of bupivacaine,the efficacy of spinal anesthesia was enhanced,and the occurrence of adverse reactions was increased.Compared with group C,the rate of successful spinal anesthesia,and the maximum level of sensory block were significantly increased,the time to reach sensory block of T5 was shortened,and the incidence of hypotension and nausea and consumption of metaraminol were increased in group E;the time to reach sensory block of T5 was significantly shortened,Apgar scores at 1 min after birth were increased (P＜0.05),and no significant change was found in the rate of successful spinal anesthesia,the first requirement for local anesthetics,the rate of time to reach sensory block ofT5≤ 15 min,and the maximum level of sensory block in group D (P＞0.05).Compared with group D,the time to reach sensory block of T5 was significantly shortened,the incidence of nausea and consumption of metaraminol were increased (P＜0.05),and no significant change was detected in the rate of successful spinal anesthesia,the first requirement for local anesthetics,the rate of time to reach sensory block of T5 ≤ 15 min,and the maximum level of sensory block in group E (P＞0.05).Conclusion The optimum dose of bupivacaine for spinal anesthesia with EVE is 9.00 mg in patients undergoing caesarean delivery.