UNLABELLED The aim of this study was to investigate the effect of light intensity on polymerization shrinkage of resin composites. The five materials tested were Lite-Fil (A) (LFA), Lite-Fil (P) (LFP), Z100, Palfique, (PAL) and Tetric (TRC). Two types of measurements were made to determine the effects of environmental temperature and light intensity. Results were analyzed by two-way analysis of variance (ANOVA) and Newman-Keuls procedure at (P<0.05). The changes in temperature and/or light intensity strongly affected composite shrinkage. The amount of shrinkage for LFP at 100% light intensity in room temperature (23 degrees C) was 1.26% and increased up to 2.29% at 60 degrees C. By raising the temperature to 37 degrees C and dropping the light intensity to 42% (459 mW/cm2), the amount of polymerization shrinkage was similar to that with full light intensity, and statistically there were no significant differences between 37 degrees C, 45 degrees C, and 60 degrees C. Lower temperatures and lower light intensities produce lower values of shrinkage. However, if the light intensity is below 28%, higher temperatures (60 degrees C) must be applied to obtain a comparable amount of shrinkage. CLINICAL RELEVANCE Pre-heating composite resin materials at body temperature (37 degrees C) is likely to improve physical properties and compensate any drop of light intensity.