An increased emphasis on the preservation of tooth structure has led to alternative cavity designs for early proximal caries. The tunnel restoration (which preserves the marginal ridge) and a conservative Class II composite resin restoration were compared in vitro for effects on tooth strength with nondestructive and destructive tests. The tunnel restoration, which is generally thought to be the more conservative of the two techniques, was actually more damaging, as shown by the stiffness, load at fracture, an d proximity to the pulp.