Background: Successful cesarean delivery anesthesia has been reported with use of small doses (5-9 mg) of intrathecal bupivacaine coadministered with opioids. This double-blind, randomized, dose-ranging study determined the ED 50 and ED 95 of intrathecal bupivacaine (with adjuvant opioids) for cesarean delivery anesthesia. Methods: Forty-two parturients undergoing elective cesarean delivery with use of combined spinal-epidural anesthesia received intrathecal hyperbaric bupivacaine in doses of 6, 7, 8, 9, 10, 11, or 12 mg in equal volumes with an added 10 μg intrathecal fentanyl and 200 μg intrathecal morphine. Sensory levels (pinprick) were evaluated every 2 min until a T6 level was achieved. The dose was a success (induction) if a bilateral T6 block occurred in 10 min; otherwise, it was a failure (induction). In addition to being a success (induction), the dose was a success (operation) if no intraoperative epidural supplement was required; otherwise, it was a failure (operation). ED 50 and ED 95 for both success (induction) and success (operation) were determined with use of a logistic regression model. Results: ED 50 for success (induction) and success (operation) were 6.7 and 7. 6 mg, respectively, whereas the ED 95 for success (induction) and success (operation) were 11.0 and 11.2 mg. Speed of onset correlated inversely with dose. Although no clear advantage for low doses could be demonstrated (hypotension, nausea, vomiting, pruritus, or maternal satisfaction), this study was underpowered to detect significance in these variables. Conclusions: The ED95 of intrathecal bupivacaine under the conditions of this study is considerably in excess of the low doses proposed for cesarean delivery in some recent publications. When doses of intrathecal bupivacaine less than the ED 95, particularly near the ED 50, are used, the doses should be administered as part of a catheter-based technique.