We present a 3-D thermal model of the Larderello geothermal field (Tuscany) to evaluate (i) the extent and contribution of the heat transfer mechanisms (conduction vs. convection) at the intermediate-upper crust levels, (ii) the variability of the heat and mass fluxes entering from below and (iii) the crucial role of the formation permeability. The model is composed by three main layers and considers the upper ten km of the crust to better constrain the simulations with the experimental data, from borehole, fluid inclusion studies and hypocentral distributions. Several sets of simulations were carried out, with different bottom boundary temperatures and different formation permeability values for the two deeper layers. The results indicate that the present temperature and pressure distribution with depth in the Larderello field requires deep reservoir rocks with higher permeabilityies than the overlying capping units and the underlying intermediate crust. Permeability values of one mDarcy for the reservoir rocks are enough to allow fluid convection, if the temperature at 10 km depth is as high as 500 ± 50 °C. The presence of localized zones with formation permeability 50-100 times higher than the surrounding rocks strongly favours the migration of over-pressurized fluids, which episodically break through the overburden, feeding the exploited geothermal fields.