The onset time and duration of action of ropivacaine during an interscalene block are not known. The potentially improved safety profile of ropivacaine may allow the use of higher concentrations to try and speed onset time. We compared bupivacaine and ropivacaine to determine the optimal long-acting local anesthetic and concentration for interscalene brachial plexus block. Seventy-five adult patients scheduled for outpatient shoulder surgery under interscalene block were entered into this double-blind, randomized study. Patients were assigned (n = 25 per group) to receive an interscalene block using 30 mL of 0.5% bupivacaine, 0.5% ropivacaine, or 0.75% ropivacaine. All solutions contained fresh epinephrine in a 1:400,000 concentration. At 1-min intervals after local anesthetic injection, patients were assessed to determine loss of shoulder abduction and loss of pinprick in the C5-6 dermatomes. Before discharge, patients were asked to document the time of first oral narcotic use, when incisional discomfort began, and when full sensation returned to the shoulder. The mean onset time of both motor and sensory blockade was <6 min in all groups. Duration of sensory blockade was similar in all groups as defined by the three recovery measures. We conclude that there is no clinically important difference in times to onset and recovery of interscalene block for bupivacaine 0.5%, ropivacaine 0.5%, and ropivacaine 0.75% when injected in equal volumes. CONCLUSIONS In this study, we demonstrated a similar efficacy between equal concentrations of ropivacaine and bupivacaine. In addition, increasing the concentration of ropivacaine from 0.5% to 0.75% fails to improve the onset or duration of interscalene brachial plexus block.