The purpose of this study was to determine patient-reported outcomes, hardware removal rates, as well as conversion to arthroplasty at mid-term follow-up following high tibial osteotomy (HTO) with a modern polyetheretherketone (PEEK) system. All PEEK system-based HTOs performed at a single institution between January 2010 and January 2016 with a minimum of 2 years follow-up were reviewed. Rates of postoperative complications, hardware removal and conversion to arthroplasty were calculated. Patient-reported outcomes including visual analog pain scale (VAS), Tegner activity scale, Lysholm, Knee Injury and Osteoarthritis Outcome Score (KOOS), and 36-Item Short Form Survey (SF-36) were obtained. Forty-seven HTOs in 43 patients (mean age: 37.6 ± 10.8 years; 30 M, 13 F) were followed for a mean of 6.0 ± 1.5 years. A total of four postoperative complications occurred in three patients, resulting in an overall complication rate of 6.4%. Hardware removal occurred in three patients at a mean of 0.73 ± 0.53 years after surgery, with overall survival free of hardware removal of 96% (95% confidence interval [CI]: 85-99) at 1 year, 94% (95% CI: 82-98) at 2 years, and 94% (95% CI: 82-98) at 8 years. Conversion to arthroplasty occurred in three patients at a mean of 3.2 ± 1.0 years after surgery, with mean survival of 100% (95% CI: 94-100) at 1 year, 100% (95% CI: 94-100) at 2 years, and 93% (95% CI: 81-98) at 8 years. VAS at rest improved, from 3.1 ± 2.5 preoperatively to 1.1 ± 1.6 postoperatively ( p < 0.001). Patients remained active, with a mean Tegner activity scale of 4.4 ± 1.4 at final follow-up and satisfactory Lysholm, KOOS, and SF-36 scores. At mid-term follow-up, medial opening-wedge HTO using a modern PEEK-based system was found to be safe, efficacious, and durable, with satisfactory outcome scores and a low rate of conversion to arthroplasty. PEEK-based implants were found to have low hardware removal rates of 6% at 5 years, which compares favorably to historic metal fixation. This is a Level III study., Competing Interests: A. J. K. reports grants from Aesculap/B. Braun, other from American Journal of Sports Medicine, personal fees and other from Arthrex, Inc., grants from Arthritis foundation, grants from Ceterix, grants from Histogenics, other from International Cartilage Repair Society, other from International Society of Arthroscopy, Knee surgery, and Orthopaedic Sports Medicine, other from Minnesota Orthopedic Society, personal fees and other from Musculoskeletal Transplant Foundation, personal fees from Vericel, personal fees from DePuy, personal fees from JRF, grants from Exactech, grants from Gemini Medical, outside the submitted work. B.A.L. reports other from Arthrex, Inc., non-financial support from Biomet, other from Clinical Orthopaedics and Related Research, other from Journal of Knee Surgery, other from Knee Surgery, Sports Traumatology, Arthroscopy, other from Orthopedics Today, non-financial support and other from Smith & Nephew, other from Stryker, personal fees from Linvatec, outside the submitted work. C.L.C. reports personal fees and non-financial support from Arthrex, non-financial support from Zimmer Biomet, outside the submitted work. M.H. reports personal fees from Moximed LLC, outside the submitted work. M.J.S. reports personal fees and non-financial support from: American Journal of Sports Medicine: Editorial or governing board, Arthrex, Inc.: IP royalties; Paid consultant; Research support, Stryker: Research support, outside the submitted work., (Thieme. All rights reserved.)