The Er:YAG is currently used as an alternative instrument for the removal of dental decay. Safe laser parameters have been found, but in order to increase the preparation speed also higher pulse energies or repetition rates are under consideration. To investigate systematically the temperature effect of these parameters and of water spray, slices of dentine were perforated with the laser radiation, exactly towards a thermocouple placed in a hole at the back side. During and after preparation temperature was monitored, and maximum temperature rise reached at the moment of perforation (Tm) was evaluated. For preparation without water irrigation Tm was in the range of 30 to 40 K, increasing slightly with pulse repetition rate (prr). For low prr (2 Hz) the same was observed for the radiant energy, however for high prr (10 Hz) the effect was inverse. When moistening the slices during preparation by a fine water spray, Tm decreases. The temperature reduction is very pronounced for low prr, leading to a temperature rise of only 2 K at 2 Hz (200 mJ). When prr is enhanced the spray becomes less effective, even when higher flow rates are chosen. With respect to temperature, combinations of low pulse energy and high repetition rate are least favorable. For safe preparations in dentine low pulse repetition rates are recommended.