Objective: To assess the influence of material selection (porcelain versus composite resin) for overlay-type restoration of endodontically treated molars and its effect on the in vitro fatigue resistance and failure mode. Method and Materials: A standardized tooth preparation was applied to 30 extracted molars, including root canal treatment, 3-mm coverage of all cusps, a mesial box 1.5 mm below the cementoenamel junction (CEJ), a distal box in enamel, a glass-ionomer base, and immediately sealed dentin. Using the Cerec machine (Sirona), all teeth were restored with an overlay of standardized thickness and occlusal anatomy. Fifteen restorations were milled in the ceramic Vita MKII block (Vident) and the other 15 using the composite resin Paradigm MZ100 block (3M ESPE). The intaglio surfaces of the ceramic restorations were etched and silanated. The intaglio surfaces of the composite resin overlays were airborne-particle abraded and silanated. Preparations were airborne-particle abraded and etched before restoration insertion. All restorations were adhesively luted with an adhesive resin (Optibond FL, Kerr) and a light-curing composite resin (Filtek Z100, 3M ESPE). Cyclic isometric chewing (5 Hz) was simulated, starting with a load of 200 N (5,000 cycles), followed by stages of 400, 600, 800, 1,000, 1,200, and 1,400 N at a maximum of 30,000 cycles each. Samples were loaded until fracture or to a maximum of 185,000 cycles. Results: MKII overlays fractured at a mean load of 1,147 N, and none of them withstood all 185,000 loading cycles (survival = 0%); with MZ100, the survival rate was 73%. With MKII, 40% of the fractures ended below the CEJ; with MZ100, only 25% did. Conclusions: Composite resin MZ100 increased the fatigue resistance of overlay-type restorations in endodontically treated molars when compared to porcelain MKII. The efficiency of the bond strategy (immediate dentin sealing) was demonstrated by the absence of adhesive failures. Keywords: Cerec, composite resin, endodontically treated molars, fatigue resistance, overlay restorations, porcelain