BACKGROUND: Locking-plate technology has renewed interest in plate fixation for treating proximal humerus fractures. Complications associated with these devices, including loss of reduction, screw cutout, and intra-articular penetration, are frequent. Establishing a second column of support may reduce complications and improve clinical outcome scores. QUESTIONS/PURPOSES: We asked whether addition of an endosteal cortical allograft strut, used as an augment to locking-plate fixation for displaced proximal humerus fractures, would reduce complications and improve clinical outcome scores. PATIENTS AND METHODS: We retrospectively reviewed the charts and radiographs of 38 patients treated by this method. All patients were evaluated with serial radiographs, as well as the Disabilities of the Arm, Shoulder, and Hand and Constant-Murley scores. There were seven two-part, 19 three-part, and 12 four-part fractures. The minimum followup was 49 weeks (average, 75 weeks; range, 49-155 weeks). RESULTS: No patient had intra-articular screw penetration or cutout. No patient had complete osteonecrosis, but one had partial osteonecrosis. The reduction was lost in one patient. The mean Disabilities of the Arm, Shoulder, and Hand score was 15 (range, 0-66.4). The mean Constant-Murley score was 87 (range, 51-95). CONCLUSIONS: Low rates of complication and high clinical outcome scores can be achieved when treating complex proximal humerus fractures with locking-plate fixation and an endosteal strut augment. LEVEL OF EVIDENCE: Level IV, therapeutic study. See Guidelines for Authors for a complete description of levels of evidence.