Statement of problem Fracture of an interim fixed partial denture (FPD) may jeopardize the success of the interim prosthodontic treatment phase and cause patient discomfort. Purpose The purpose of this study was to compare the fracture toughness of a polymethyl methacrylate (PMMA) resin and a bis-acryl composite (BAC) resin reinforced with stainless steel wire, glass fiber, and polyethylene fiber. Material and methods Four groups (n=13) of each of the 2 materials were prepared for the single-edge notch 3-point-bending test. Three groups had the different reinforcements, and the group without reinforcement served as the control. Using a universal testing machine, peak load to fracture was recorded and fracture toughness (KIC) was calculated in MNm-1.5. Median KIC values were compared by means of nonparametric ANOVA (Kruskal-Wallis test, a=.05). Results For the controls, the fracture toughness for PMMA resin (KIC=27.9) was significantly lower (P<.01) than for BAC resin (KIC=31.2). Glass fibers and stainless steel wire reinforcements produced significantly higher fracture toughness for both PMMA (KIC=34.4, P<.01, and KIC=39.0, P<.001, respectively) and BAC resin (KIC=42.3, P<.001, and KIC=44.0, P<.001, respectively), but the polyethylene fibers did not (KIC=25.8, P>.10, for PMMA resin and KIC=33.1, P>.10, for BAC resin). There was no significant difference between the fracture toughness of the wire and glass fiber reinforcements for both interim materials (P>.10 in both instances). Conclusions Of the 3 reinforcement methods evaluated, wire and glass fiber reinforced the PMMA and BAC resin materials best. (J Prosthet Dent 2008;99:461-467)