Purpose: The aim of this study was to determine the effect of long-term water immersion on the flexural properties of fiber-reinforced composite. Materials and Methods: Continuous, woven, silanized E-glass fibers and woven silica fibers were used to reinforce heat-cured and autopolymerized denture base polymers. Fibers were oriented at a 45- degree angle to the long axis of the test specimens. Control specimens were unreinforced. Dry test specimens and those stored in water for up to 180 weeks were tested with a 3- point loading apparatus. Results: Ultimate transverse strength and flexural modulus of unreinforced and fiber-reinforced composite test specimens decreased during water storage (P 0.001, analysis of variance). Post hoc analysis revealed that after the storage of 4 weeks no statistically significant reduction occurred. Conclusion: The results of this study suggest that the ultimate transverse strength of the fiber-reinforced composite made from E-glass fibers is reduced by approximately 27% compared to the dry fiber-reinforced composite. The majority of the reduction occurred during 4 weeks of storage in water and remained approximately at that level for 180 weeks.