Purpose: To compare and evaluate knee laxity and functional outcomes between autologous bone graft and silicatesubstituted calcium phosphate (Si-CaP) in the treatment of tunnel defects in 2-stage revision anterior cruciate ligament reconstruction (ACLR). Methods: This prospective, randomized controlled trial was conducted between 2012 and 2015 with a total of 40 patients who underwent 2-stage revision ACLR. The tunnels were filled with autologous iliac crest cancellous bone graft in 20 patients (control group) and with Si-CaP in the other 20 patients (intervention group). After a minimum follow-up period of 2 years, functional outcomes were assessed by KT-1000 arthrometry (side-to-side [STS] difference), the Tegner score, the Lysholm score, and the International Knee Documentation Committee score. Results: A total of 37 patients (follow-up rate, 92.5%) with an average age of 31 years were followed up for 3.4 years (range, 2.2-5.5 years). The KT-1000 measurement did not show any STS difference between the bone graft group (0.9 AE 1.5 mm) and the Si-CaP group (0.7 AE 2.0 mm) (P ¼ .731). One patient in the intervention group (5%) had an STS difference greater than 5 mm. Both groups showed significant improvements in the Tegner score, Lysholm score, and International Knee Documentation Committee score from preoperative assessment to final follow-up (P .002), without any difference between the 2 groups (P ! .396). Complications requiring revision occurred in 4 control patients (22%) and in 2 patients in the intervention group (11%) (P ¼ .660). No complications in relation to Si-CaP were observed. Conclusions: Equivalent knee laxity and clinical function outcomes were noted 3 years after surgery in both groups of patients. Si-CaP bone substitute is therefore a safe alternative to autologous bone graft for 2-stage ACLR. Level of Evidence: Level I, prospective, randomized controlled clinical trial. See commentary on page 186 R upture of the anterior cruciate ligament (ACL) is one of the most common knee injuries, and a marked increase in anterior cruciate ligament reconstructions (ACLRs) has occurred over the past 20 years. 1,2 Because of the rise in operations, the number of revision procedures is also increasing