Objectives: To evaluate the effects of thermal cycling on flexural properties of composites reinforced with two differently sized silica-glass fibers suggested for prosthodontic applications. Methods: Acid-washed, woven, fused silica-glass fibers (Enka AG, Obernburg, Germany), were heat-treated at 500ºC, silanized and sized with one of two sizing resins (linear poly(butyl methacrylate) (PBMA) or cross-linked poly(methyl methacrylate) (PMMA)). Subsequently the fibers were incorporated in a polymer matrix. Two groups of specimens with fibers (polymer matrix + fiber content of 36.9wt%) and one control group without fibers were prepared. The flexural properties of dry and water saturated (wet) specimens were determined by 3-point bend testing before thermal cycling. Another set of wet specimens was thermally cycled in water baths (12 000 cycles, 5 ºC/55 ºC, dwell time 30s), followed by 3-point bend testing. The data were analyzed by t-test (p<0.05). SEM micrographs were taken of the fibers and test specimens. Results: (Table) Different lowercase letters indicate statistical differences (within rows). Conclusion: Water saturation did not alter the flexural strength of test specimens made of unreinforced polymer, or of silica glass fiber reinforced polymer. However, after thermocycling less flexural strength reduction (30%) of the dry specimens was observed for the fiber composite sized with PBMA than for the composite sized with cross-linked PMMA (50%). Due to the flexible structure of the linear PBMA above the low Tg glass transition temperature, the interfacial PBMA-sizing polymer acts as stress-bearing component for the high interfacial stresses during thermal cycling. The interfacial cross-linked PMMA-sizing, however, acts as a rigid component and therefore during bending causes adhesive fracture due to different thermal expansion of fibers and matrix during the fatigue process of thermal cycling.