OBJECTIVE 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 C(5-6) 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. CONCLUSIONS 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.