PURPOSE: Interscalene brachial plexus block (interscalene block) complications usually depend on the dose administered. The objective of this study was to determine whether ultrasound-guided interscalene block with a 5-mL dose of 0.75% ropivacaine would have sufficient analgesic efficacy after shoulder arthroscopic surgery when compared with a 10-mL dose. METHODS: Patients undergoing arthroscopic rotator cuff repair surgery (n = 60) were assigned randomly to one of two groups receiving 5 mL (Group 5) or 10 mL (Group 10) of 0.75% ropivacaine. Ultrasound-guided interscalene block was performed using the in-plane technique, and general anesthesia was administered. Time to first analgesic request was recorded, and the following issues were assessed in the postanesthesia care unit at six, 12, 24, and 48 hr postoperatively: postoperative pain as determined by a visual analogue scale (VAS), patient satisfaction, hemidiaphragmatic paralysis, other block-related complications, and postoperative nausea and vomiting (PONV). RESULTS: There was no significant difference between groups in time to first analgesic requirement (median [interquartile range] for Group 5: 16 [12-48] hr, Group 10: 18 [12-48] hr; P = 0.907). The postoperative pain VAS score was similar in both groups. The incidence of hemidiaphragmatic paralysis on postoperative chest x-ray was 33% in Group 5 and 60% in Group 10 (P = 0.035). However, the incidences of other block-related complications, PONV, and patient satisfaction were not significantly different between groups. CONCLUSION: Interscalene block performed under ultrasound guidance with 0.75% ropivacaine 5 mL showed analgesic efficacy similar to that with 0.75% ropivacaine 10 mL, but with a lower incidence of hemidiaphragmatic paralysis.