In 1991, a hydrogeochemical surveillance program was set up on Mount Etna, involving periodic measurement of several chemico-physical parameters in springs and wells along the eastern and southwestern flanks of the volcano. Significant increases in the temperature of these waters were observed some months before the 1991–1993 eruption. The energy responsable for this heat anomaly represents less than 0.3% of the total thermal energy released by the following eruption. The contribution from thermal conductivity of the rocks is negligible. The ascent of the gases released from the magma determined the transfer of energy from the magma to the aquifers of Etna. The energy that produced the thermal anomaly in the groundwaters is one order of magnitude lesser than the minimum estimated energy transported by the gases released from the erupted magma. Continuous monitoring of groundwater temperatures and other parameters may not only predict eruptions in the future, but also permit an estimate of the volume of magma involved.