This study investigated the influence of pins on the fracture resistance of three core materials. Two or four stainless steel pins were incorporated in either amalgam, composite resin, or alloy-reinforced glass ionomer specimens. Half of the pins were surface-treated with mercury, Panavia EX resin, or hydrochloric acid before they were incorporated in the respective materials. The pins were oriented in a direction relative to the tensile stress/axis of the specimen: parallel/perpendicular (PL/PR), perpendicular/parallel (PR/PL), or perpendicular/perpendicular (PR/PR). ANOVA tested significant differences in diametral tensile strength among materials, in number of pins, in pin orientations, in surface treatments, and in other interactions. Incorporation of pins weakened amalgam the most, followed by composite resin. Pins did not weaken amalgam-reinforced glass ionomer. Pin orientation improved the fracture resistance of some specimens by two times that of the controls. Orientation of pins parallel to the tensile stress was most favorable. As the number of pins increased, the fracture resistance of amalgam significantly decreased. Acid treatment of the pin surface enhanced the bond with composite resin. Both treatments resulted in significant improvement in fracture resistance.