BACKGROUND: Due to the highly organized tissue and avascular nature of the rotator cuff, rotator cuff tears have limited ability to heal after the tendon is reinserted directly on the greater tubercle of the humerus. Consequently, retears are among the most common complications after rotator cuff repair. Augmentation of rotator cuff repairs with patches has been an active area of research in recent years to reduce retear rate. HYPOTHESIS: Graft augmentation with 3D collagen could prevent retears of the repaired tendon and improve tendon-bone healing in moderate to large rotator cuff tears. STUDY DESIGN: Randomized controlled study; Level of evidence, 2. METHODS: A prospective, randomized controlled study was performed in a consecutive series of 112 patients age 50 to 85 years who underwent rotator cuff repair with the suture-bridge technique (58 patients, control group) or the suture-bridge technique augmented with 3-dimensional (3D) collagen (54 patients, study group). All patients were followed for 28.2 months (range, 24-36 months). Visual analog scale score for pain, University of California Los Angeles (UCLA) shoulder score, and Constant score were determined. Magnetic resonance imaging was performed pre- and postoperatively (at a minimum of 24 months) to evaluate the integrity of the rotator cuff and the retear rate of the repaired tendon. Three patients in each group had biopsies at nearly 24 months after surgery with histological assessment and transmission electron microscopy. RESULTS: A total of 104 patients completed the final follow-up. At the 12-month follow-up, the UCLA shoulder score was 28.1 ± 1.9 in the study group, which was significantly better than that in the control group (26.9 ± 2.1, P = .002). The Constant score was also significantly better in the study group (87.1 ± 3.2) than in the control group (84.9 ± 4.2, P = .003). However, at the final follow-up, no significant differences were found in the UCLA shoulder scores (29.4 ± 1.9 in the control group and 30.0 ± 1.6 in the study group, P = .052) or Constant scores (89.9 ± 3.2 in the control group and 90.8 ± 3.5 in the study group, P = .18). In terms of structural integrity, more patients in the study group had a favorable type I retear grade (18/51) than in the control group (10/53) ( P = .06). The postoperative retear rate was 34.0% in the control group and 13.7% in the study group, thus indicating a significantly lower retear rate in the study group ( P = .02). Biopsy specimens of the tendon-bone interface in 6 patients revealed more bone formation and more aligned fibers with larger diameters in the study group than in the control group. No intraoperative or postoperative complications were noted in either group. CONCLUSION: 3D collagen augmentation could provide effective treatment of moderate to large rotator cuff tears, providing substantial functional improvement, and could reduce the retear rate. This technique could also promote new tendon-bone formation, thus exerting a prominent effect on tendon-bone healing.