BACKGROUND: Combined spinal epidural anesthesia (CSE) often produces a more extensive spinal block than expected. This study was designed to evaluate the effects of CSE on subarachnoid block in patients undergoing lower extremity surgery. METHODS: Thirty-three patients who undergone lower extremity surgeries were randomly allocated to three groups of 11 patients each. Using needle through needle technique, all patients received a subarachnoid injection of hyperbaric 0.5% bupivacaine 1.6~2.0 ml through a 25G Whitacre spinal needle. Group 1 received no extradural injection for 25min, but group 2 and 3 received extradural saline 10 ml and bupivacaine 10 ml 5min after the subarachnoid injection, respectively. Levels of sensory and motor block were assessed at 4, 6, 8, 10, 15, 20, and 25 min after subarachnoid injection. RESULTS: The median values of maximum sensory block level were T7 in all groups. Levels of sensory blockade and the time to onset of maximum sensory blockade were similar among the three groups. There was no significant difference in the degree of motor block among three groups. CONCLUSIONS: This study suggests that extradural saline 10 ml or 0.5% bupivacaine 10 ml which injected 5min after subarachnoid injection does not significantly influence the level of subarachnoid block in lower extremity surgical patients. However, further study is required to declare the safety or optimal dose of extradural injection during CSE.