This study tested the comparative cyclic fatigue of six prefabricated post systems (Flexi-Post (Essential Dental Systems, South Hackensack, NJ), Flexi-Anterior (Essential Dental Systems), Vlock (Brasseler USA, Inc, Savannah, GA), Parapost (Whaledent/Coltenen, New York, NY), Cytco (L.D. Caulk, Milford, DE), and Access post (Essential Dental Systems). Cyclic fatigue testing was obtained with an in vitro fatigue machine that simulated clinical fatigue conditions. An instantaneous applied force of 5.0 pounds was applied to each post specimen and data was recorded when post breakages occurred. If no breakages occurred, then the test was stopped after 2,000,000 repetitions. Failures only occurred with the Cytco and Vlock post designs. The Flexi-Post, Flexi-Anterior, Parapost and the Access post all had greater resistance to cyclic fatigue with no failures. This experimental design may simulate biting forces and stresses seen for posts in a clinical situation.