The Phlegrean Fields is an active caldera structure, located on the periphery of Naples (Italy). After the last eruptive event (the Monte Nuovo eruption in 1538), periodic episodes of unrest have characterized the evolution of this volcanic district, involving seismic activity and slow ground motion (bradyseism). During these episodes of unrest, some remarkable changes have also affected the composition of the hydrothermal fluids discharged at La Solfatara fumarolic field. These unrest phenomena result from the complex interaction between magma chamber, hydrothermal fluid circulation, and country rocks undergoing thermal and mechanical stresses. In order to make an effective hazard assessment in such a densely populated area as the Phlegrean Fields, we must first reach a better understanding of the mechanism driving bradyseismic activity and determine the relation between ground deformation and hydrothermal fluid circulation. In this work, we present some results of numerical modeling of both the hydrothermal fluid circulation at La Solfatara, and of its effects on rock deformation. The modeling results show that periods of intensified magmatic degassing can explain many features of the recent crises of unrest at Phlegrean Fields. (C) 2004 CNR. Published by Elsevier Ltd. All rights reserved.