This study evaluated the load, applied to the marginal ridge, required to produce failure in Class II posterior composite resin restorations with four different preparation designs. In group 1, the preparation had an extension through the occlusal groove. The other three groups employed a proximal box-only (slot) preparation. Group 2 preparations had facial and lingual retention grooves that extended from the gingival floor to the occlusal surface; group 3 preparations were slots without grooves; and group 4 preparations were slots without grooves and with unsupported proximal enamel. After the restorations were thermocycled, their marginal ridges were flattened and loaded to failure. Mean (SD) failure loads were 438 (73) N in group 1; 383 (52) N in group 2; 297 (72) N in group 3; and 281 (63) N in group 4. Mean failure loads of groups 1 and 2 were not significantly different from each other but were significantly greater than failure loads for groups 3 and 4. Mean failure loads of groups 3 and 4 were not significantly different.