Although interscalene brachial plexus block (ISBPB) is often used to provide anesthesia for arthroscopic shoulder surgery, its selective analgesic effect, provided by low-dose local anesthetic, has not been studied. We hypothesized that ISBPB using a low volume and low concentration of bupivacaine can provide effective postoperative analgesia for shoulder surgery without producing significant sensory or motor block elsewhere. In this double-blind study, 30 outpatients scheduled to undergo shoulder arthroscopy were randomly assigned to receive either an ISBPB with 10 mL 0.125% bupivacaine with epinephrine 1:400,000 (n = 15) or 10 mL of normal saline (n = 15). The block was performed preoperative, prior to a standardized general anesthetic. Postoperative pain scores, imorphine and oral analgesic consumption, recovery profile, and patient satisfaction were recorded. In the ISBPB group, verbal analog pain scores within 120 minutes after surgery were lower, morphine consumption in the postanesthesia care unit was significantly lower (2.7+/-2.6 mg vs 9.5+/-5.2 mg), the time to postoperative administration of the first systemic or oral analgesic was significantly longer (141+/-182 minutes vs 13+/-10 minutes), the degree of motor and sensory block 120 minutes after surgery was minimal, time to reach hospital discharge criteria was earlier, and patient satisfaction with postoperative analgesia at 24-hour follow-up was greater. Thirty-three percent of the patients receiving ISBPB did not require any analgesic prior to hospital discharge. Interscalene brachial plexus block with low-dose bupivacaine is a useful and selective analgesic technique for outpatient shoulder arthroscopic surgery.