The purpose of this study was to compare the probability of failure, expressed as B10 strength, of three systems of machined zirconia ceramics (Lava, DC-Zirkon, and Cercon) with a zirconia-reinforced aluminum ceramic (In-Ceram Zirconia). Ten rectangular specimens of each material were subjected to a three-point flexural strength test. The B10 strength of Lava was significantly lower than that of DC-Zirkon but significantly higher than that of Cercon and In-Ceram Zirconia. The B10 strength of Cercon and In-Ceram Zirconia were not significantly different from one another. The machined zirconia ceramics appeared to fail primarily with intergranular fracture.