Piton de La Fournaise (Riunion Island) had shown 8 months of high activity during 2003, including four eruptions and one intrusion. The monitoring networks, provided by the Volcanological Observatory of Piton de La Fournaise, show two different kinds of behavior: strong and rapid deformations linked with the magma injections and a slight continuous intereruptive inflation of the summit area. Inversed modeling based on the deformation data, using a three-dimensional elastostatic boundary element code, combined with seismicity provides new insights into the volcanic feeding system involved in the 2003 activity. (1) The 2003 eruptions are fed from a pressurized magma chamber located beneath the summit at 300 m above sea level. (2) A vertical path was activated between the magma chamber and the surface during the December 2002 collapse, allowing the magma to rise into it and leading to the initial summit eruption in May 2003. (3) The August and September 2003 lateral eruptions had been fed by dikes intruding along the same vertical pathway, at an estimated velocity of 1.1 to 1.3 m/s before intruding the flank into the rift zones at a velocity of 0.2 to 0.8 m/s. This last point suggests a strong structural control on the dike pathway.