This single-centre prospective randomised trial aimed to assess the superiority of operative fixation compared with non-operative management for adults with an isolated, closed humeral shaft fracture. 70 patients were randomly allocated to either open reduction and internal fixation (51%, n=36/70) or functional bracing (49%, n=34/70). 7 patients did not receive their assigned treatment (operative n=5/32, non-operative n=2/32); results were analysed based upon intention-to-treat. The primary outcome measure was the DASH score at 3 months. Secondary outcomes included treatment complications, union/nonunion, shoulder/elbow range of motion, pain and health-related quality of life (HRQoL). At 3 months, 66 patients (94%) were available for follow-up; the mean DASH favoured surgery (operative 24.5, non-operative 39.4; p=0.006) and the difference (14.9 points) exceeded the MCID. Surgery was also associated with a superior DASH at 6wks (operative 38.4, non-operative 53.1; p=0.005) but not at 6 months or 1yr. Brace-related dermatitis affected 7 patients (operative 3%, non-operative 18%; OR 7.8, p=0.049) but there were no differences in other complications. 8 patients (11%) developed a nonunion (operative 6%, non-operative 18%; OR 3.8, p=0.140). Surgery was associated with superior early shoulder/elbow range of motion, and pain, EuroQol and SF-12 Mental Component Summary scores. There were no other differences in outcomes between groups. Surgery confers early advantages over bracing, in terms of upper limb function, shoulder/elbow range of motion, pain and HRQoL. However, these benefits should be considered in the context of potential operative risks and the absence of any difference in patient-reported outcomes at 1yr.