Temperature variations in the clinical range may adversely affect bonded composite resin restorations. The aim of this study was to determine the effect of thermal cycling on fracture strength and microleakage in posterior teeth restored with bonded composite resin. Fourteen matched pairs of maxillary premolars were prepared with an MOD slot of uniform dimensions and restored with a posterior composite resin bonded to enamel and dentin (Scotchbond and P-30). One member of each pair was randomly assigned to an uncycled control group; the other to a test group which was cycled between 5° and 55°C. After cycling the test group, both groups of teeth were stained for microleakage with silver nitrate solution, then fractured by occlusal loading in a universal testing machine. The fracture strength of teeth in the groups was compared by Student's matched pairs t test, and microleakage was compared by the Wilcoxon signed-rank test. A significant decrease in fracture strength (P=0.02) of the thermal-cycled teeth was seen when compared with the control teeth. Extensive microleakage was found among both control and thermalcycled teeth. The results of this in vitro study suggest that variations in temperature in the clinical range may reduce the fracture strength gained with bonded posterior composite resins. Polymerization shrinkage may produce clinically significant microleakage even before thermal cycling of the teeth.