Several engineers and researchers have highlighted that in many cases, even for moderate seismic events, non-structural elements and, in detail, masonry infills have suffered severe damages. This circumstance required interventions with considerable repair costs and disruption to service use of buildings. Otherwise, generally, the seismic design of structures omits the influence of such elements in the estimation of the seismic performance of the structure. The contribution of infills doesn’t influence only the need of interventions after an earthquake but can also be responsible of a variation of the dynamic response of the frames, as it can modify the entities of the displacements with respect to the estimated ones. In the cases, for example, of retrofitting interventions which include the installation of passive devices, the reduction of displacements may inhibit the activation of such devices and/or may reduce the amount of dissipated energy. Therefore, the numerical simulation of the infilled frames and the estimation of their modal parameters are the first step in the design procedure of new structures or of interventions for the improvement of the seismic performance of existing buildings. The paper deals with this topic, by proposing a simplified model able to take into account both frame and infills. A numerical analysis is performed to evaluate the influence of the infills features on the global response of the infilled frames.