Background: Operations require collaboration between surgeons, anaesthetia professionals, and nurses. The aim of this study was to determine whether intraoperative briefings influence patient outcomes. Methods: In a before-And-After controlled trial (9 months baseline; 9 months intervention), intraoperative briefings were introduced in four general surgery centres between 2015 and 2018. During the operation, the responsible surgeon (most senior surgeon present) briefed the surgical team using the StOP? protocol about: progress of the operation (Status), next steps (Objectives), possible problems (Problems), and encouraged asking questions (?). Differences between baseline and intervention were analysed regarding surgical-site infections (primary outcome), mortality, unplanned reoperations, and duration of hospital stay (secondary outcomes), using inverse probability of treatment (IPT) weighting based on propensity scores. Results: In total, 8256 patients underwent surgery in the study. Endpoint data were available for 7745 patients (93.8 per cent). IPT-weighted and adjusted intention-To-Treat analyses showed no differences in surgical-site infections between baseline and intervention (9.8 versus 9.6 per cent respectively; adjusted difference (AD)-0.15 (95 per cent c.i.-1.45 to 1.14) per cent; odds ratio (OR) 0.92, 95 per cent c.i. 0.83 to 1.15; P = 0.797), but there were reductions in mortality (1.6 versus 1.1 per cent; AD-0.54 (-1.04 to-0.03) per cent; OR 0.60, 0.39 to 0.92; P = 0.018), unplanned reoperations (6.4 versus 4.8 per cent; AD-1.66 (-2.69 to-0.62) per cent; OR 0.72, 0.59 to 0.89; P = 0.002), and fewer prolonged hospital stays (21.6 versus 19.8 per cent; AD-1.82 (-3.48 to-0.15) per cent; OR 0.87, 0.77 to 0.98; P = 0.024). Conclusion: Short intraoperative briefings improve patient outcomes and should be performed routinely.