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Visceral pain decreases in cesarean patients under spinal anesthesia when the dose of local anesthetic is increased. However, larger doses of local anesthetic are associated with higher sensory blocks. We hypothesized that the addition of fentanyl could reduce the dose of bupivacaine necessary to achieve adequate surgical anesthesia.Two double-blinded, sequential, prospective studies were performed on 120 patients. In the preliminary study, the patients received 8, 10, or 12 mg of 0.5% hyperbaric bupivacaine intrathecally. In the second, main study, they received each bupivacaine dose with 10 μg of fentanyl. Each group consisted of 20 patients, and the groups were identified as B8, B10, B12, BF8, BF10, and BF12. Sensory and motor block, intraoperative pain defined by visual analogue scale (VAS), muscle relaxation, and side effects were assessed. We also measured the sensory and motor recovery and the onset of pain in the postanesthesia care unit (PACU).Maximal block level and incidence of high block (≥T1) were higher in the 12-mg groups. There was intraoperative pain in 35% of the B8 patients and 20% of the B10 patients, but none in the B12 patients and all fentanyl groups. Incidences of other side effects were not different. The addition of fentanyl to bupivacaine significantly delayed the onset of postoperative pain and sensory recovery, but motor recovery time did not change with additional fentanyl.The optimal dose of hyperbaric bupivacaine to produce surgical anesthesia was 12 mg, which was accompanied by high sensory block. With the addition of 10 μg of fentanyl, the dose of bupivacaine could be reduced to 8 mg in spinal anesthesia for cesarean delivery.