Purpose: To evaluate the failure patterns of teeth restored with both cast metal and fiber posts after fatigue cycling. Materials and Methods: Thirty-five crownless human teeth subdivided into 5 groups were subjected to 2 3 106 100-N fatigue cycles at 8 Hz under 37°C water. Results: No root fatigue fractures were recorded. High microleakage values were found in specimens that survived fatigue. No statistically significant relationships were found for fatigue (P > .4), fracture strength (P > .8), and microleakage (P > .1). Conclusions: Cast-metal and fiber posts undergo insidious microscopic adhesive failures at similar levels. Increasing the post diameter and/or post stiffness could improve the core stabilization.