The effect of cuspal coverage on the in vitro fracture resistance of teeth restored with indirect composite resin inlays was examined. After standardized preparation differing only in cavity width, two groups of teeth were restored with indrect composite resin restorations. Two further groups of teeth were prepared in a similar manner, except that the buccal and palatal cusps were reduced by 2 mm, and cuspal-coverage indirect composite resin restorations placed. In another group, indirect composite resin restorations with only palatal cuspal coverage were placed. One group of intact teeth acted as a control. Forces were applied to the restored teeth by means of a 4-mm steel bar in a Universal testing machine. For each of the preparation designs assessed, the fracture load for onlays was greater than that for equivalent inlays; the onlay fracture strength was equivalent, for some preparation designs, to that of sound teeth.