BACKGROUND: Intraoperative injuries to the medial collateral ligament are often unrecognized and failure to appropriately manage ligament loss may result in knee instability and loosening. QUESTIONS/PURPOSES: We compared the functional scores in patients with iatrogenic injury to the medial collateral ligament (MCL) treated with additional constraint to those without. METHODS: We retrospectively reviewed the records of all 1478 patients (1650 knees) who underwent primary TKA between 1998 and 2004. Thirty-seven patients (2.2%) had recognized intraoperative injury to the MCL; the remaining 1441 patients (1613 knees) served as controls. We attempted to repair the ligament in 14 patients; increased prosthetic constraint over that planned was used in 30 of the 37 patients. We determined Knee Society scores (KSS) in all patients. Three patients were lost to followup. The minimum followup was 36 months (average, 54 months; range, 36-120 months). RESULTS: The mean KSS for all MCL injury knees for pain and function averaged 81 and 74 points, respectively, compared with 91 and 87 for the control group. However, in the 30 knees in which the MCL insufficiency was treated with increased constraint, the mean scores for pain and function increased to 88 and 83 points, respectively. Four of the seven patients treated without increased prosthetic constraint were revised for instability; no revisions for instability were performed in the 37 patients treated with additional constraint. CONCLUSIONS: Recognition of MCL injury during TKA is crucial, since using nonstabilizing inserts was associated with residual instability requiring revision. LEVEL OF EVIDENCE: Level IV, therapeutic study. See Guidelines for Authors for a complete description of levels of evidence.