PURPOSE: Various implant materials have been used in medial, opening-wedge high tibial osteotomy (HTO) including traditional metal and modern polyetheretherketone (PEEK) implants. The purpose of this study was to compare metal and PEEK implants and determine safety, varus deformity correction, as well as short- to mid-term hardware removal and arthroplasty rates. METHODS: HTO performed with metal and PEEK implants were reviewed between 2000 and 2015 at two institutions with a minimum of 2 years follow-up. Postoperative complications, radiographic measures, and osteotomy union were compared between groups using Kruskal-Wallis and Fisher's exact testing. Survival free of hardware removal and arthroplasty was compared between groups using Kaplan-Meier testing. Risk factors for HTO conversion to arthroplasty were examined using Cox proportional hazards regression. RESULTS: Ninety-five HTOs were performed in 90 patients (59 M, 31 F) using 50 metal and 45 PEEK implants. Mean follow-up was 4.2 years (range 2.0-16.5). Two metal and two PEEK HTO patients experienced nonunions, resulting in revision HTO at a mean of 1.0 years postoperatively (range 0.4-1.4 years). Both implant groups demonstrated similar, significant improvements in coronal deformity, with mean angulation improving from 6.0 degrees and 5.4 degrees varus preoperatively to 1.1 degrees and 1.0 degrees valgus postoperatively for the metal (p < 0.01) and PEEK groups (p < 0.01), respectively. 2- and 5-year hardware removal-free survival was 94% and 94% for PEEK, which was significantly superior to 80% and 73% observed for metal (p = 0.02). 2- and 5-year arthroplasty-free survival was similar for the metal (98% and 94%) and PEEK groups (100% and 78%) (n.s.). HTO performed for focal cartilage defects was observed to demonstrate decreased arthroplasty risk (HR 0.36, p = 0.03) when compared to HTO performed for osteoarthritis. CONCLUSIONS: Both metal and PEEK implants were found to be effective in obtaining and maintaining coronal varus deformity correction, with 88% overall arthroplasty-free survival at 5 years. Metal fixation demonstrated a higher rate of hardware removal while HTO performed for medial compartment osteoarthritis predicted conversation to arthroplasty. LEVEL OF EVIDENCE: III.