Aim: To examine the influence of the accuracy of fit on mouthguard retention in relation to mouthguard outline location. Materials and Methods: Mouthguards were fabricated on a fully dentate maxillary cast with 4-mm thick ethylene vinyl acetate sheets using a well-dried cast (VD) and on a wet cast (VW) with a vacuum-forming machine. Mouthguards were also made with a pressure-forming machine (P). The mouthguard outline was set at 4 mm from the cervical margin (control, C), the palatal outline was set at the cervical margin (Pa), and the buccal outline was extended to the buccal fold (Bu). A dislodging force perpendicular to the occlusal plane was applied at the first molar on the universal testing machine. Results were analyzed using ANOVA with Scheffe's test (P .05). Results: Using the VW, the retentive forces were statistically smaller at Pa compared to Ca or Bu (C: 58 ± 17 gf, Pa: 36 ± 7 gf , Bu: 50±8gf) (P .05). There were no statistical differences between VD (C: 116±27gf, Pa: 86±15gf, Bu: 92±7gf) and group P (C: 133±31gf, Pa: 139±24gf, Bu: 136±17gf). Conclusions: The results indicate that mouthguard retention is closely related to the accuracy of fit at the cervical undercut area rather than the outline location. Keywords: accuracy of fit, cast, design, mouthguard, outline, retention