Infill walls are not generally taken into account in structural analysis due to their complex behavior at seismic actions. As it is known, they increase the stiffness as well as the lateral load capacity of the system. Sometimes, in fill walls may have window and door openings in their planes. In the present study, behavior of Reinforced Concrete (RC) frames with infill walls, which have openings, is investigated under cyclic lateral loadings. Location and size of the openings in the in fill wall are selected as investigation parameters. Test specimens are constructed and experimentally analyzed. The in fill wall changes the behavior of the frames under cyclic lateral loads significantly. Location and size openings in the infill wall are two main parameters that affect the behavior of the in fill walls as well as the frame. The test results clearly show that the contribution of the in fill wall to the behavior of RC frame diminishes significantly when the opening ratio is larger than 9%. Therefore, the effect of the opening in the infill wall must be taken into account in the structural modeling when the opening ratio is larger than 9%. (C) 2017 Sharif University of Technology. All rights reserved.