Purpose: The aim of this study was to compare the retentive properties of an attachment with a nylon insert and an attachment with a polyetherketoneketone (PEKK) insert on two-implant mandibular overdentures during simulated masticatory loadings and insertion/removal cycles. Materials and Methods: Two attachment systems with nylon inserts or PEKK inserts were investigated, including nylon/ low, nylon/medium, nylon/high, PEKK/x-low, PEKK/low, PEKK/medium, and PEKK/high. The canine region of edentulous mandibular overdenture models was retained with two implant analogs through two different stud attachments at interimplant angulations of 0 and 20 degrees. The simulated mastication and insertion/ removal cycles were 400,000 cyclic loadings and 1,080 times of insertion/removal. Wear patterns and deformations on the attachment surface were analyzed with scanning electron microscopy. Results: At 0 and 20 degrees, initial retentive force was highest in nylon/high and lowest in PEKK/x-low. Final retentive force was highest in PEKK/high and PEKK/medium at 0 and 20 degrees, respectively, and PEKK/x-low was lowest at both angulations. At 0 degrees, nylon/low, nylon/medium, PEKK/x-low, and PEKK/low displayed a significant increase in retention (P .05), and nylon/high and PEKK/medium showed a significant decrease in retention (P .05), except PEKK/high, which exhibited no significant decrease (P > .05). At 20 degrees, nylon/low and PEKK/x-low increased retention significantly (P .05), and nylon/medium, nylon/high, PEKK/ low, PEKK/medium, and PEKK/high decreased retention significantly (P .05). Patrices of both attachment systems barely showed detectable abrasion, while matrices revealed severe wear and surface deformation. Nylon inserts particularly displayed more noticeable wear and deformation than PEKK inserts. Conclusion: Within the limitations of this investigation, the attachment system with a PEKK insert indicated less abrasion and retention change than the attachment system with a nylon insert on both paralleled implants and axially tilted implants under 400,000 cyclic loadings and 1,080 insertion/removal cycles for 1 year. Keywords: attachments, implant overdentures, nylon, retention, PEKK