The majority of peripheral nerve injuries (PNIs) affect the upper extremity, 1,2 with the ulnar nerve being the most commonly affected. 1,3,4 Common mechanisms include motor vehicle accidents and penetrating trauma. [1][2][3] These injuries disproportionally affect young men, and can result in functional impairment of the upper extremity. [1][2][3][4][5] Traditionally, PNIs are treated using tensionfree primary repair, 3,6,7 particularly resulting from penetrating trauma. 6,8 Several factors may lead to poor prognosis, including proximal lesions, delayed repair, and patient age. 6 Proximal nerve neurotmesis and axonotmesis injuries are particularly challenging, resulting in sensory and Background: Proximal ulnar nerve lacerations are challenging to treat because of the complex integration of sensory and motor function in the hand. The purpose of this study was to compare primary repair and primary repair plus anterior interosseous nerve (AIN) reverse end-to-side (RETS) coaptation in the setting of proximal ulnar nerve injuries. Methods: A prospective cohort study was performed of all patients at a single, academic, level I trauma center from 2014 to 2018 presenting with isolated complete ulnar nerve lacerations. Patients underwent either primary repair (PR) only or primary repair and AIN RETS (PR + RETS). Data collected included demographic information; quick Disabilities of the Arm, Shoulder and Hand questionnaire score; Medical Research Council score; grip and pinch strength; and visual analogue scale pain scores at 6 and 12 months postoperatively. Results: Sixty patients were included in the study: 28 in the PR group and 32 in the RETS + PR group. There was no difference in demographic variables or location of injury between the two groups. Average quick Disabilities of the Arm, Shoulder and Hand questionnaire scores for the PR and PR + RETS groups were 65 ± 6 and 36 ± 4 at 6 months and 46 ± 4 and 24 ± 3 at 12 months postoperatively, respectively, and were significantly lower in the PR + RETS group at both points. Average grip and pinch strength were significantly greater for the PR + RETS group at 6 and 12 months. Conclusion: This study demonstrated that primary repair of proximal ulnar nerve injuries plus AIN RETS coaptation yielded superior strength and improved upper extremity function when compared with PR alone. (Plast.