The purpose of this study was to elucidate the mechanism of action by which an epidural top-up reinforces anesthesia in combined spinal epidural anesthesia. Thirty patients scheduled to undergo lower limb orthopedic surgery were randomly allocated to three groups of 10 patients each. In all patients, a 16-gauge Tuohy needle was introduced into the epidural space. Using the needle through needle technique, each patient received a subarachnoid injection of 10 mg plain bupivacaine 0.5% through a long 27-gauge Whitacre spinal needle introduced into the subarachnoid space through the Tuohy needle. After withdrawal of the spinal needle, an epidural catheter was introduced into the epidural space. After the maximum level of sensory block after the subarachnoid injection had been established, an epidural top-up with 10 mL bupivacaine 0.5% (Group 1) or 10 mL saline (Group 2) was administered; patients in Group 3 received no epidural top-up. The maximum level of sensory block was then assessed for an additional 30 min. After the epidural top-up the maximum level of sensory block increased significantly by 4.8 +/- 1.6 segments in Group 1 and 2.0 +/- 2.0 segments in Group 2. In Group 3 there was a nonsignificant increase of 0.3 +/- 0.5 segments. Intergroup comparisons showed that this increase in Group 1 was significant compared with those in Groups 2 and 3, and that the increase in Group 2 was significant compared with that in Group 3. We conclude that the mechanism of action by which an epidural top-up reinforces anesthesia in combined spinal epidural anesthesia can be explained partly by an epidural volume effect and partly by an effect of the local anesthetic itself.