Background: An increase in the maximum level of sensory blockade (MLSB) following an epidural top-up in combined spinal epidural anesthesia (CSE) may be achieved by a volume effect as the volume of local anesthetic compresses the dural sac, by a local anesthetic effect, or by a combination of both effects. This study was conducted to investigate the contribution of each of these effects. Methods: Sixty patients scheduled for lower limb surgery under CSE were randomly allocated to one of three groups of twenty patients each. Using the needle-through needle technique, all patients received a subarachnoid dose of 10 mg hyperbaric 0.5% bupivacaine. At 30 min after subarachnoid injection, an epidural top-up with saline 10 ml (group II) or 0.5% bupivacaine 10 ml (group III) was administered; patients in group I received no epidural top-up. The level of sensory blockade was assessed at 5, 10, 15, 20, 25, 30 min after subarachnoid injection and at 5, 10, 15, 20, 25, 30 min after epidural top-up. Results: There was no significant difference in the MLSB and the onset time of MLSB among group I-III. Conclusions: We concluded that the epidural top-up with saline 10 ml or 0.5% bupivacaine 10 ml which injected at 30 min after subarachnoid injection did not significantly increase the level of subarachnoid block in lower extremity surgical patients.