Purpose: The effect of glass-fiber reinforcement and water storage on the fracture toughness (KIC) of polymer-based provisional crown and fixed partial denture (FPD) materials was investigated. Materials and Methods: Five unreinforced single-edged, notched control specimens and five test specimens reinforced with unidirectional Eglass fibers (Stick) were fabricated from three dimethacrylate-based provisional materials and one monomethacrylate-based provisional material. The specimens were stored in water at 37°C for 1, 7, 30, or 60 days. Specimens were loaded in three-point bending at a cross-head speed of 0.1 mm/s. Mode I plane-strain KIC was calculated using the maximum load, and results of the two groups were compared. The water storage effect on KIC with time was also evaluated. Results: The KIC of provisional materials reinforced with glass fibers (range 7.5 to 13.8 MNm-1.5) was significantly higher than that of unreinforced materials (range 1.3 to 3.1 MNm-1.5), by a factor of 4.4 to 5.5. A small, gradual decrease of KIC in reinforced specimens occurred with aqueous storage, but it was not statistically significant. Conclusion: The KIC of polymer-based provisional crown and FPD materials was significantly increased when they were reinforced with unidirectional E-glass fibers. Water storage for up to 2 months still left the reinforced materials with KIC values in excess of 7 MNm-1.5. Hence, their performance was satisfactory.