PurposeThis study determined the fracture toughness of four core buildup materials.Materials and MethodsSingle‐edge notch, bar‐shaped specimens conforming to the American Society for Testing Materials standard E‐399 were fabricated for a high copper amalgam alloy, two composite resins, and a glass ionomer buildup material. The specimens were stored in air for 1 week and then tested in three‐point bending mode with an Instron Universal Testing Machine (Instron Corporation, Canton, MA).ResultsFracture toughness values obtained were as follows: Fluorocore (composite resin; Caulk, Milford, DE), 1.54 MN · m−1.5; Ti‐Core (composite resin and titanium; Essential Dental Systems, New York, NY), 1.34 MN · m−1.5; Valiant Ph.D. (amalgam; Caulk), 1.29 MN · m−1.5; and Coreshade Glass lonomer Base Cement (Shofu Inc, Kyota, Japan), 0.55 MN · m−1.5.ConclusionsGlass ionomer materials are probably unsuitable as core buildup materials because of their relatively low fracture toughness. Fluorocore, Ti‐Core, and amalgam all had fracture toughness values significantly greater than th