In this study, infill walls, which are commonly used nowadays, were examined using linear elastic method to investigate the positive and negative effects on steel frame systems under lateral loads. The most common models in structural applications were reviewed, and structural steel and gas concrete were selected for the load resisting frame system and the infill material respectively. The purpose of this study is to show that infill walls have advantages and disadvantages not only under the collapse load, but also under service loading. An equivalent diagonal compression bar model was used for the elastic analysis of infill walls in two-story, two-bay planar frames under lateral loads; and a total number of seven models were analyzed for the combinations of steel frames with, without, and partial infill walls. The strength calculations of the equivalent diagonal compression bar were based on the mechanical and geometric properties of the gas concrete. Effects of symmetrically and asymmetrically placed infill walls on the lateral displacement, rigidity, and natural vibration period of the frame were investigated as well as short column and soft story behaviors. The structures were modeled and analyzed using the SAP2000 structural analysis program.