Thermal percolation has been considered a cause of a number of the flaws occurring in connection with resin fillings. However, in fillings expanded against the cavity walls by water absorption, elastic stresses counteract the formation of marginal gaps. The purpose of the present work was 1) to study the relationship between temperature reduction and size of marginal gaps and 2) to investigate the effect of a rise in temperature on the gaps formed under subsequent cooling. Fillings made in extracted human teeth were studied. The brands investigated were Adaptic, Blendant, Concise, Opotow, Sevriton Simplified, and Swedon. After closure of the initial gaps by water absorption expansion the fillings were polished and examined microscopically under water. Marginal gaps between filling and tooth were measured between 37 and 2°C. The effect of a rise in temperature was investigated by heating the fillings to SO or 60°C, with subsequent measurement of gap size at 2°C. It was found that the fillings could be cooled through a certain temperature range without marginal gaps being formed. The size of the gaps at 2°C was but little affected by previous heating to 50°C. Heating to 60°C increased the gap size at 2°C only in unfilled resins.