OBJECTIVES: To evaluate the influence of the ferrule effect (1) and the fiber-post placement (2) on the fracture resistance of endodontically treated teeth subjected to cyclic fatigue loading. METHODS: 40 extracted single-rooted upper pre-molars were sectioned at the CEJ (groups a-b) or 2mm above the CEJ (groups c-d), and subsequently endodontically treated. After 24-hour water storage at 37°C, specimens were restored according to four build-up approaches (n=10 per group): a. NF-NP (no ferrule, no post), b. NF-P (no ferrule, fiber-post), c. F-NP (ferrule, no post), d. F-P (ferrule, fiber post). RelyX Posts (3M-ESPE) were used in groups NF-P and F-P, and were cemented with Panavia F 2.0 (Kuraray). A standardized composite core was built, after which specimens were restored with an all-ceramic crown (IPS Empress CAD, Ivoclar-Vivadent) Specimens were fatigued by exposure to 1,200,000 cycles using a chewing simulator (Willytech). All specimens that survived fatigue loading were fractured using a universal loading device (Micro-Tester, Instron). A two-way analysis of variance was used to determine the statistical significance of the factors ferrule and post on fracture resistance. RESULTS: Only one NF-NP specimen failed under fatigue. The ferrule effect significantly enhanced the fracture resistance of the restored teeth, regardless the use of a post (p=.003). F-NP obtained the highest fracture resistance (758.52±121.89N), which was not significantly different from F-P (647.58±132.95N); NF-NP presented the lowest fracture resistance (361.52±151.69N). For all groups, only 'repairable' failures were recorded. Conclusions: Avoiding extra-removal of sound tooth structure, rather than placing a fiber post, can protect endodontically treated teeth against catastrophic failure. However, when any ferrule can be preserved, a fiber-post may improve the retention and fatigue resistance of the restoration.