Purpose: This study compares the onset time and quality of posterior approach interscalene brachial plexus block produced by 0.25% levobupivacaine and 0.25% bupivacaine. Methods: Sixty adult patients undergoing open or closed shoulder surgery were enrolled in this double-blind, randomized study, and they were randomly allocated to receive 40 ml of 0.25% levobupivacaine (Group L, n = 30) or 0.25% bupivacaine (Group B, n = 30). The patients were assessed at 5 min intervals after local anesthetic injection in order to determine loss of shoulder abduction and loss of pinprick sensation in the C5-6 dermatomes. The mean onset time of motor and sensory block and onset time of complete motor and sensory block were documented in both groups. Results: In both groups, mean onset time of sensory block was <5 min and mean onset time of complete sensory block was <25 min. The onset times for sensory block and complete sensory block were not statistically different between the groups (P > 0.05). In both groups, mean onset time of motor block was <10 min but the mean onset time of complete motor block was <30 min. The onset times of motor block and complete motor block were not statistically different among the groups (P > 0.05). After the injection of the local anesthetic, 27% of Group L and 87% of Group B had complete motor block. Four patients in Group L had no motor block. Conclusion: We conclude that 0.25% levobupivacaine and 0.25% bupivacaine have similar motor and sensory block onset times and qualities when used in posterior approach interscalene brachial plexus block, and provide comfortable anesthesia and analgesia for shoulder surgery. © 2009 Japanese Society of Anesthesiologists.