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Background A bilateral oblique subcostal transverse abdominis plane block may help provide perioperative analgesia and reduce opioid use in patients undergoing sublay mesh hernia repair, but its clinical value is unclear.Methods In a single-centre, prospective, placebo-controlled, double-blind study, patients scheduled for sublay mesh hernia repair were randomized to receive oblique subcostal transverse abdominis plane blocks with either 60 ml of 0.375% ropivacaine (n=19) or isotonic saline (placebo, n=17). The primary outcome was patient-controlled total morphine consumption at 8:00 p.m. on the second postoperative day (POD), while secondary outcomes included the total morphine consumption during the post-anesthesia care unit stay and the occurrence of adverse events.Results Total morphine consumption at 8:00 p.m. on the second POD was higher in patients receiving ropivacaine (39 mg, IQR 22, 62) compared with placebo (24 mg, IQR 7, 39), p value = 0.04. In contrast, the ropivacaine group received 2 mg less morphine during the post-anesthesia care unit stay (4 mg, IQR: 4, 9 mg vs 2 mg, IQR: 2,6 mg, p = 0.04). Patients receiving ropivacaine used more morphine (8:00 p.m. on the first POD until 8:00 a.m. on the second POD: 8 mg, IQR: 4, 18 mg vs 2 mg, IQR: 0, 9 mg, p = 0.01) and reported higher maximum pain scores since the last assessment (8:00 a.m. on the second POD: 5, IQR: 4, 7 vs 4, IQR: 3, 5, p = 0.03). There were no differences in adverse events between groups.Conclusions Bilateral oblique subcostal transverse abdominis plane blocks in patients undergoing sublay mesh hernia repair were not associated with a prolonged reduction in patient-controlled total morphine consumption in the evening of the second POD in this study. Rebound pain might explain the additional excess opioid required by the ropivacaine group.