Mass-mortality events of the temperate coral Cladocora caespitosa were recorded in the Gulf of La Spezia (NW Mediterranean) in the summers of 1997, 1998 and 1999. At the end of the summer 1999, 60% of the colonies found in 1997 were damaged and more than 50% of the damaged colonies were completely dead. Complete death of the largest colonies caused a modal shift of colony size-frequency distribution towards the smallest size classes. In order to verify experimentally the hypothesis that mass-mortality was primarily induced by a seawater temperature increase of up to 4 °C for several weeks, as proposed by several Authors, nubbins of the coral were maintained in aquaria at a temperature of 28 °C, i.e., 4 °C higher than the normal maximum summer value. After three weeks of treatment, the nubbins showed a mortality pattern similar to that observed in the field, with the gradual necrosis of tissue to leave only the bare skeleton. Similarly, nubbins long-term exposed to the summer temperature of 24 °C showed signs of stress. In situ tissue necrosis and loss, first from coenosarcs, then from polyps, started from different parts of the colonies simultaneously. In addition, C. caespitosa did not bleach during the thermal stress, which is different from the heat-induced mass mortality events in tropical corals.