Objective: To determine whether temperature affects the polymerization shrinkage of composite resin. Method and Materials: Volumetric shrinkage of a microfill composite was measured using a video-imaging device. The material was cooled to 4°C or heated to 37°C, 54°C, or 68°C. Room-temperature (20°C) composite was used as the control. The composite was light activated using a quartz-tungsten-halogen curing device. Polymerization shrinkage was recorded at 1-minute intervals for 10 minutes after the beginning of photoinitiation. The data were subjected to ANOVA, Fisher's PLSD test, and linear regression analysis (α = 0.05). Results: Volumetric shrinkage ranged from 0.2% to 4.7%. Shrinkage of the composite at 37°C was similar to that at room temperature. Preheating the composite to 54°C or 68°C significantly increased shrinkage. The apparent polymerization shrinkage of the refrigerated composite was significantly less than that of the control, although this might have been an artifact of the test method. Regression analysis revealed a strong correlation between temperature and volumetric shrinkage. Conclusion: Preheating composite to relatively high temperatures (54°C or 68°C) to increase its flow and adaptation causes an increase in volumetric shrinkage. At body temperature, composite shrinkage is similar to that at room temperature. Keywords: composite resin, polymerization shrinkage, preheating