Two groups of maxillary premolars with Class I cavities were prepared with one marginal ridge thickness to a width of 1.0 mm. One group was restored with amalgam and the other group with composite resin. Two groups of Class II cavities also were prepared and restored with amalgam and composite resin. The teeth in the four treatment groups were subjected to a centric load at the marginal ridge until fracture occurred. A two-factor analysis of variance revealed a statistically significant difference between the class of preparation (Class I preparations were weaker than were Class II preparations) but differences in strength between restorative materials (composite resin versus amalgam) and the interaction effect (class of preparation x restorative materials) were not found to be statistically significant (α = .05). A Newman-Keuls sequential range test found no statistically significant differences in strength between groups (α = .05). Fracture patterns were observed under scanning electron microscope. Fractures extended into the enamel and into the material in the Class 1 composite resin, Class I amalgam, and Class II composite resin restorations. Fractures extended only into the material in Class II amalgam restorations.