This multi-centre single-blind randomised control trial compared outcomes in patients with acute displaced mid-shaft clavicle fractures treated either by primary open reduction and plate fixation (ORPF), or non-operative treatment (NT). Two-hundred patients were randomised to receive either ORPF or NT. Functional assessment was conducted up to one-year using DASH, SF-12 and Constant scores (CS). Union was evaluated using radiographs and CT. Rate of non-union was significantly reduced after ORPF (1 following ORPF, 16 following NT, odds ratio=0.07, 95% CI=0.01–0.50, p=0.0006). 7 patients had delayed-union after NT. Group allocation to ORPF was independently predictive of development of non-union. DASH and CS were significantly better in the ORPF group 3-months post-surgery, but not at one-year (mean DASH = 6.2 after NT versus 3.7 after ORPF, p=0.09; mean CS = 86.1 after NT versus 90.7 after ORPF, p=0.05). Group allocation was not predictive of one-year outcome. Non-union was the only factor independently predictive of one-year functional outcome. There were no significant differences in time off work or subjective scores. Five patients underwent revision for complications after ORPF. 10 patients underwent metalwork removal. Treatment cost was significantly greater after ORPF (p=0.001). ORPF reduces rate of non-union compared with NT and is associated with better early functional outcomes. Improved outcomes are not sustained at one-year. Differences in functional outcome appear to be mediated by prevention of non-union from ORPF. ORPF is more expensive and associated with implant-related complications not seen with NT. Our results do not support routine primary ORPF for displaced mid-shaft clavicle fractures.