This study determined the effect of 10 years of water immersion on the properties of fiber-reinforced resin composites (FRC). E-glass and silica fibers were used to reinforce heat-cured and autopolymerized acrylic resin polymers. Control specimens were unreinforced. Test specimens stored in water for up to 10 years were tested by the 3-point test. The flexural strength and elasticity of the specimens decreased during water immersion (P .001, analysis of variance). After 10 years, the reductions in flexural strength and modulus of E-glass FRC were 24% and 21%, respectively; for silica FRC, reductions were 47% and 46%, respectively; and for controls, reductions were 24% and 11%, respectively. E-glass FRC showed a smaller reduction in strength than silica FRC.