Purpose: The aim of this study was to determine the effect of water on the flexural properties of fiber-reinforced denture base polymers. Materials and Methods: Continuos woven silanized electrical glass, or 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 48 weeks were tested with a three-point loading apparatus. The surface of the fibers of the test specimens stored dry or 48 weeks in water were analyzed with a scanning electron microscope to evaluate the degree of adhesion between fibers and polymer matrix. Results: The ultimate transverse strength of unreinforced and reinforced denture base polymers decreased during 48 weeks' storage in water (P 0.05, one-way analysis of variance, n = 5), and most of this reduction occurred during the first 4 weeks of storage in water. The flexural modulus of the unreinforced test specimens decreased significantly (P 0.001), whereas there was less, if any, change in the flexural modulus of the fiber-reinforced test specimens. Scanning electron microscopic examination revealed no differences in adhesion of E-glass fibers to the polymer matrix when the specimens stored in water wre compared with those stored dry. Reduced adhesion between the silica fibers and matrix was observed after 48 weeks' storage in water. Conclusion: The results of this study suggest that the ultimate transverse strength of the E-glass fiber-reinforced test specimens decreased 14% and that of the silica fiber-reinforced test speicmens decreased 36% after 48 weeks of storage in water.