Discrete gravity data were acquired at Mount Etna over a period lasting more than a year and spanning the July--August 2001 flank eruption. The gravity measurements were made using a Scintrex CG-3M gravimeter along two profiles of Etna's gravity network. The gravity data indicate that during the second half of 2000, a mass increase of 2 ¿ 1010 kg took place about 1.5 km below the summit craters zone. It is thought to be due to a magma accumulation. A comparable mass decrease was detected during a 4-month period encompassing the eruption which is thought to be the result of withdrawal of the same magma. The gravity data also show that a mass decrease of 2.5 ¿ 1011 kg occurred between January and 19 July 2001 (2 days after the start of the eruption), below the southeastern sector of the volcano, within a volume around 2.5 km below sea level. This decrease was followed by a sudden increase that, in 14 days, partially compensated the previous variation. Since many tensional earthquakes took place during the first half of 2001, within the same source volume of the inferred mass decrease, it is reasonable to assume that the variation observed before the eruption reflects the formation of new voids by tectonic tensile stresses below the volcano edifice. Magma was emplaced along the new formed path from a deep storage to higher levels of Etna's plumbing system, setting off the eruption and causing the gravity increase observed soon after its start. Vulcanological, petrological, and ground deformation studies agree with the assumption of a batch of magma coming directly from a deep storage to have supplied most material extruded during the eruption.