A 69 year record (1922–1991) of heat release has been obtained from the subglacial geothermal system of the Grímsvötn caldera within the Vatnajökull ice cap. The data were derived from in situ measurements of the volume of meltwater accumulated in the caldera lake, subtracting climatically induced melting. The overall fluctuations in the heat flux are closely related to volcanic activity and are dominated by a main pulse of 11,600 MW, caused by a major eruption in 1938, gradually declining to 1600 MW in 1976–1982. Heat extracted from the roof of a magma chamber, with the aid of hydrothermal convection, may have given a basic contribution to the heat flux of 1500 to 2000 MW (an upper bound). The variable part of the heat flux (from 0 to 10,000 MW) was released from magma erupted at the base of the glacier and from shallow intrusions. The total heat released over the period 1922 to 1991 was (8.1±1.6)×1018J, equivalent to the energy released by the solidification and cooling of 2.1±0.4 km³ of basaltic magma. The contribution to the total heat flux was 45% (max.) from a magma chamber, 35% (min.) from shallow intrusions, and 20% from eruptions. This implies that magma at the roof of a chamber solidified and cooled at the rate of 1.2−1.6×107 m³ a−1 or about 1 km³ over the last 69 years. Heat release at Grímsvötn was probably more intense in the 19th century when volcanic eruptions were more frequent.