This study compared the resistance to intermittent loading of teeth with tapered, individually cast posts and cores or prefabricated parallel-sided posts (Para-Post, Whaledent) and composite cores. Bovine teeth mounted in acrylic resin blocks with a simulated periodontium were used for the study. Two conditions were studied. One group received individually fabricated posts and cores, the second had titanium prefabricated posts and resin composite cores placed. All teeth receiving posts and cores had cast crown restorations placed. Teeth were intermittently loaded using a force of 250 N twice each second at an angle 45 degrees to the long axis until failure occurred. Teeth with prefabricated posts and resin composite cores showed a significantly higher resistance to intermittent loading than did teeth with tapered, individually cast posts and cores. Additional restored teeth were sectioned and the adaptation of the posts to the root were measured. The adaptation of the prefabricated posts was better than that of tapered, individually cast posts.