Objective: To characterize and evaluate treatment options for medial rectus muscle (MR) injury associated with functional endoscopic sinus surgery (FESS). Design: Retrospective interventional case series Participants: A total of 30 cases were gathered from 10 centers. Methods: Cases of orbital MR injury associated with FESS surgery were solicited from members of the American Society of Ophthalmic Plastic and Reconstructive Surgery (ASOPRS) through an e-mail discussion group. Main Outcome Measures: Variables assessed included patient demographics, computerized tomography and operative findings, extent of MR injury and entrapment, secondary orbital/ocular injuries, initial and final ocular alignment and ductions, and interventions. Results: A spectrum of MR injury ranging from simple contusion to complete MR transection, with and without entrapment, was observed. Four general patterns of presentation and corresponding injury were categorized. Conclusions: Medial rectus muscle injury as a complication of FESS can vary markedly. Proper characterization and treatment are important, particularly with reference to the degree of direct MR injury (muscle tissue loss) and entrapment. Patients with severe MR disruption can benefit from intervention but continue to show persistent limitation of ocular motility and functional impairment. Prevention and early recognition and treatment of these injuries are emphasized.