An incremental buildup technique is advocated for placing posterior composite resin restorations. This technique is preferred because it assures complete polymerization of large restorations and also reduces the problem of polymerization shrinkage, which can adversely affect the seal at the gingival margin of class II and large class III restorations. Measurements of tensile bond strengths between layers have been published with conflicting results. This study measured the fracture toughness of layered posterior composite resin within the interfacial layer and in a direction perpendicular to the layering. A compact test-specimen geometry conforming to the ASTM standard E399-83 was used. Three groups were tested: Group A specimens were bulk-cured in one complete increment; group B specimens were incrementally cured in 1.2 +/- 0.2 mm layers perpendicular to the plane of fracture; and group C specimens were incrementally cured with the interface between layers parallel to and in the plane of fracture. The microfilled composite resin showed no significant difference in the mean fracture toughness from the bulk-cured control regardless of layering technique. The hybrid composite resin showed a significant decrease in fracture toughness for fractures occurring in the interfacial layer, and fractures perpendicular to layering showed no significant difference in fracture toughness compared with the bulk-cured control.