The purpose of the present work was to investigate the wall-to-wall polymerization contraction of restorative resins placed in acid etched cavities and to study the effect of water absorption and temperature changes on the formation of marginal gaps on such fillings. Fillings made in extracted human teeth were examined microscopically. Before filling the cavities including 1.5-2 mm peripheral enamel were etched with 35% H3PO4. When polished and examined immediately after setting, fillings of a number of brands showed no marginal gaps. Gaps due to continued polymerization were formed around some of these fillings after 1 day's storage in water at 37 degrees C. These gaps did not close as a consequence of hygroscopic expansion of the fillings. Around fillings after 1 day's storage in water at 37 degrees C. These gaps did not close as a consequence of hygroscopic expansion of the fillings. Around fillings of the brands where no gaps present immediately after setting, gaps were formed by cooling from 37 to 23 degrees C. One day's storage in water at 37 degrees C before polishing generally reduced the frequency of marginal gaps both directly and after cooling. Heating of the fillings increased the risk of gaps being formed by a subsequent cooling.