STUDY DESIGN: This is a retrospective clinical study. OBJECTIVES: To compare the efficacy of lidocaine and bupivacaine for the ultrasound-guided lumbar medial branch block in chronic low back pain. SUMMARY OF LITERATURE REVIEW: There is no study for comparison of the efficacy between lidocaine and bupivacaine for the medial branch block. MATERIALS AND METHODS: From August 2011 to May 2013, 186 patients were assigned 0.5% lidocaine(n=136) or 0.25% bupivacaine(n=45) for the ultrasound-guided lumbar medial branch block. All procedures have been performed by the same operator, and 23G, 10 cm needle was placed and drug was injected under ultrasound guide. To target medial branches from lumbar spinal nerve, the groove at the root of transverse process and the base of superior articular process has been identified on transverse scan. Patients were evaluated by pre- and post-interventional(1 hour) Visual Analog Scale and analyzed statistically. RESULTS: Reduction of VAS score in bupivacaine group is significantly greater than that in lidocaine group and post-interventional VAS score in bupivacaine group is significantly lower than that in lidocaine group through analysis of covariance test with adjusted pre-interventional VAS score. In multivariate analysis, while age, sex and treatment level were not significant factors, pre-interventional VAS score and the kind of drug were significant factors. Severe pain before treatment and bupivacaine was indicator of better result. Bupivacaine group reduced pain score in the VAS 2.285 more than lidocaine group with adjustment with other factors. CONCLUSIONS: Bupivacaine is more effective than lidocaine in the reduction of pain after ultrasound-guided lumbar medial branch blocks in posterior facet joint syndrome.