ABSTRACT: In this work the effects of the soil on the seismic response of existing RC structures have been checked, with reference to two Hospital buildings placed in Tuscany (Italy). The Hospital has been the object of a joint agreement between the Regional Government of Tuscany and the University of Florence, and, consequently, a careful experimental campaign has been performed, providing detailed information both on the building and on the construction soil. The soil profile has been checked through three different geological investigations, which have provided different stratigraphies; the obtained soil profiles have been compared to the soil classification provided by the Italian (NTC 2008), and European (Eurocode 8), Technical Codes. The seismic input has been represented by assuming an ensemble of ground motions compatible to the elastic spectrum provided by the Code for the assumed soil-type, and by assuming an alternative ensemble of ground motions representing the bedrock of the area and filtering them though the soil layers. Two different assumptions have been made about the soil behavior, so obtaining two alternative ensembles of filtered seismic spectra. The seismic capacity of the building structures has been represented by an inelastic non-linear analysis, which has included all the main geometrical and mechanical properties. The seismic response of the buildings have been determined by means of spectral analysis, crossing the capacity curve of the structure with the alternative spectral demand found in the different hypotheses assumed for the soil behavior.