Continuous interscalene block is an approved modality for postoperative pain control, but it may cause hemidiaphragmatic paresis. In this study we aimed to determine whether continuous supraclavicular block would provide postoperative analgesia comparable to that of continuous interscalene block and reduce the incidence of hemidiaphragmatic paresis. Patients scheduled for open rotator cuff repair were randomly allocated to receive continuous interscalene (n = 38) or supraclavicular block (n = 37). Both participants and assessing clinicians were blinded to the group allocation. The primary endpoint was the mean pain intensity 24 h after the surgery. Postoperative mean (SD) pain scores at 24 h were similar in the supraclavicular and interscalene groups (2.57 (1.71) vs 2.84 (1.75) respectively; p = 0.478). The incidence of complete or partial hemidiaphragmatic paresis was lower in the supraclavicular group at 1 h after admission to the postanaesthetic care unit and 24 h after the surgery [25 (68%) vs 38 (100%); p = 0.001 and 14 (38%) vs 27 (71%) respectively; p = 0.008]. Continuous supraclavicular block provided comparable analgesia compared with interscalene block with a reduced incidence of complete or partial hemidiaphragmatic paresis for 24 h following surgery.