Reinforced concrete frames infilled with masonry panels without proper seismic design can protect or potentiate the risk of structures when subject to earthquakes. This paper presents two case studies of existing reinforced concrete buildings with masonry infill panels located in Portugal in different seismic zones according with the EuroCode 8. The RC buildings were studied with and without the infill panels using non-linear dynamic analysis, comparing the different seismic response in terms of global storey displacements, inter-storey drifts, storey shear and ductility demands in the columns. The results show that in one building it is safe and conservative not to take the panels into account, however in the case with irregular infill distribution in elevation the results are significantly different and the presence of the infills plays and important role.