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Objectives: Septic arthritis after anterior cruciate ligament (ACL) reconstruction is a rare but potentially devastating complication. The imaging findings associated with such infections are not well described or quantified. The purpose of this study was to describe and quantify the frequency of the characteristic magnetic resonance imaging (MRI) findings of infection following ACL reconstruction., Methods: Seventeen cases were identified from 2010 to 2018, confirmed to meet established definitions for deep infections, and had an MRI obtained within ten days of diagnosis of infection. Two board-certified musculoskeletal radiologists evaluated and scored MRI studies with a predetermined set of imaging characteristics. Cohen's kappa (±) was utilized to determine the extent of agreement between the radiologists. Associations between graft type and microbiologic results and MRI findings were assessed., Results: The overall infection rate was 0.42%, diagnosed at a median 35 days postoperatively (range 9-411). Grafts were retained in 11 of 17 (64.7%) infected cases and 9 patients (52.9%) required repeat irrigation and debridement. Common imaging findings included complex lamellated effusions (k ​= ​0.86), fluid within the femoral tunnel (k ​= ​0.91), and femoral tunnel resorption (k ​= ​0.83). Subcutaneous edema and edema surrounding the femoral tunnel were also identified in the majority of cases. There was no evidence of cortical destruction of the tunnels or chondrolysis. All acute infections demonstrated complex lamellated effusions, whereas all simple effusions were seen in chronic cases. No associations were found between index ACL graft type or microbiologic results and MRI findings., Conclusion: Interrater agreement for common imaging findings, including effusion pattern, edema surrounding the graft tunnels and tunnel resorption was near perfect within infected ACL reconstructed knees. MRI can provide valueable information regarding postoperative infection after ACL reconstruction., Level of Evidence: Level IV- epidemiological observational study., Competing Interests: Declaration of competing interest Blake C. Meza, Niv Marom, Harry Greditzer IV, and Eric Bogner declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Robert G. Marx declares the following financial interests/personal relationships which may be considered as potential competing interests: Robert G. Marx reports a relationship with Mend Nutrition Inc. that includes: consulting or advisory and equity or stocks. Robert G. Marx reports a relationship with International Society of Arthroscopy Knee Surgery and Orthopedic Sports Medicine that includes: board membership. Robert G. Marx is recipient of royalties for book published by Springer and Desmos Health. If there are other authors, they declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2025 The Authors. Published by Elsevier Inc. All rights reserved.)