BACKGROUND: This is a semidouble-blind randomized controlled trial comparing the clinical and radiographic outcomes of fixation of displaced 3- and 4-part proximal humerus fractures with an intramedullary nail versus a locking plate. METHODS: Seventy-nine patients aged 45 to 81 years (mean 66.5 years) were randomized to open reduction and osteosynthesis with either locking nails or plates. The primary outcome was the Disabilities of the Arm, Shoulder, and Hand (DASH) score at 2 years after surgery. The secondary outcomes were the Constant score (CS), Oxford shoulder score (OSS), visual analog scale for pain at rest and during activity, and complication and reoperation rates. The patients were assessed at 6 weeks, 12 weeks, 6 months, 1 year, and 2 years after inclusion. Physiotherapists who were blinded to the choice of implant performed assessments at 6 months and 2 years. Patients were also blinded to the choice of implant. RESULTS: Three patients were lost to follow-up, leaving data from 38 patients in each group available for analysis. There were no statistically significant differences in the DASH score, CS, OSS, or pain score at any time. The DASH score at 2 years was 12.7 (95% confidence interval [CI] 8.8-17.0) for the nail group and 10.0 (95% CI 5.6-16.1) for the plate group (p = 0.48). Twelve (32%) patients underwent reoperation in the nail group, whereas 2 (5%) patients underwent reoperation in the plate group (p = 0.006). Fourteen (37%) patients in the nail group and 4 (11%) patients in the plate group experienced complications (p = 0.05). CONCLUSIONS: There were no statistically significant differences in patient-reported outcomes or function 2 years after surgery for fixation of displaced 3- and 4-part part proximal humerus fractures with nails or plates. However, there were more complications and reoperations in the nail group. LEVEL OF EVIDENCE: Therapeutic Level I. See Instructions for Authors for a complete description of levels of evidence.