This study evaluated the fracture resistance of cusp replacing direct resin composite restorations (DCR) in premolars that had been previously filled with amalgam mesial-occlusal-distal (MOD) restorations and compared their fracture resistance with those made on sound dentin and intact teeth. Recently extracted human premolars with either MOD amalgam restorations or sound/intact ones were selected for the study. Cavities with cusp reduction were made for the following groups: (a) Group 1: DCRs on previously amalgam-affected dentin (n=11), (b) Group 2: DCRs on sound dentin (n=10), and (c) Group 3: intact premolars (n=9). Teeth in Groups 1 and 2 were restored with a 3-step etch and rinse adhesive (Quadrant Unibond) and filled with hybrid composite (Clearfil Photo Posterior). All specimens were thermocycled for 5000cycles (5-55 degrees C). The buccal cusps of the teeth were loaded until fracture under compression at 45 degrees to the long axis of the teeth in a universal testing machine (1mm/min). Data (N) were statistically analyzed using one-way ANOVA and Student's t-test (=0.01). Intact teeth (Group 3) showed significantly higher fracture resistance (893 +/- 196) compared to both restored groups (p<0.01). No significant difference was found between the DCRs made on amalgam-affected dentin (Group 1: 607 +/- 166) and sound dentin (Group 2: 588 +/- 183) (p>0.01). More than half of the teeth of Groups 2 and 3 showed unrepairable fractures with pulp exposure.