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Ryosuke Kawanishi,1 Kaori Yamamoto,2 Yoko Tobetto,2 Kayo Nomura,2 Michihisa Kato,2 Ritsuko Go,2 Yasuo M Tsutsumi,3 Katsuya Tanaka,3 Yoshitsugu Takeda4 1Department of Anesthesiology, Tokushima University Hospital, Tokushima, 2Division of Anesthesiology, Tokushima Red Cross Hospital, Komatsushima, 3Department of Anesthesiology, Institute of Health Bioscience, University of Tokushima Graduate School, Tokushima, 4Division of Orthopedics, Tokushima Red Cross Hospital, Komatsushima, Japan Purpose: To determine the effects of intravenous and perineural dexamethasone on the duration of interscalene brachial plexus block (ISB) with ropivacaine in patients undergoing arthroscopic shoulder surgery. Patients and methods: In this prospective, randomized, placebo-controlled trial, patients presenting for arthroscopic shoulder surgery with an ISB were randomized to receive ropivacaine 0.75% (group C), ropivacaine 0.75% plus perineural dexamethasone 4 mg (group Dperi), or ropivacaine 0.75% plus intravenous dexamethasone 4 mg (group Div). The primary outcome was the duration of analgesia, defined as the time between performance of the block and the first request for analgesic. Results: Thirty-nine patients were randomized. The median times of sensory block in groups C, Dperi, and Div were 11.2 hours (interquartile range [IQR] 8.0–15.0 hours), 18.0 hours (IQR 14.5–19.0 hours), and 14.0 hours (IQR 12.7–15.1 hours), respectively. Significant differences were observed between groups Dperi and C (P=0.001). Kaplan–Meier analysis for the first analgesic request showed significant differences between groups Dperi and C (P=0.005) and between groups Dperi and Div (P=0.008), but not between groups C and Div. Conclusion: Perineural but not intravenous administration of 4 mg of dexamethasone significantly prolongs the duration of effective postoperative analgesia resulting from a single-shot ISB with ropivacaine 0.75%. Keywords: ropivacaine, interscalene nerve block, dexamethasone, postoperative pain, perineural