Purpose: The aim of this study was to investigate the influence of fatigue on the fracture strength of In-Ceram (Vita Zahnfabrik), Optimal Pressable Ceramics (Opc, Jeneric Pentron), and IPS Empress (Ivoclar-Vivadent) in both wet and dry environments. Materials and Methods: Twenty-six crown shapes measuring 8.0 mm in diameter and 8.5 mm in height were fabricated for each ceramic system. For each ceramic system, 10 specimens were tested for fracture strength without fatiguing. A second group (8 specimens) was submitted to a fatigue and fracture test in dry conditions, and a third group (8 specimens) was fatigued and fractured in a wet enviro nment using a mechanical testing machine (Instron). The results were statistically analyzed using a Mann-Whitney test. Results: The results indicated that: (1) The fracture strength for In-Ceram was significantly stronger than IPS Empress. No difference was found between In-Ceram and Opc, and Opc and IPS Empress. (2) The strength of the three ceramic systems decreased significantly after fatiguing in both dry and wet environments compared with the nonfatigued specimens. No difference was found between fatiguing in dry and wet environments. (3) For the three systems fatigued in a dry environment and then fracture tested, In-Ceram and Opc were significantly stronger than IPS Empress, but no difference was found between the three systems fatigued in a wet environment. Conclusion: Significant differences in the fracture strengths of the different systems investigated may be seen that result from both the nature of the system and the environment in which the specimens were fatigued.