Strengthening of reinforced concrete frames by cast-in-place reinforced concrete infills is commonly used in practice. Sometimes, window or door openings have been left in the infill wall due to architectural necessities during the strengthening of RC framed buildings. But behavior of partially infilled reinforced concrete frames under lateral cyclic loads is not known fully. The purpose of this research is to investigate the behaviour of ductile reinforced concrete (RC) frames strengthened by introducing partial infills under cyclic lateral loading. One-bay, one-story, 1/3 scale nine test specimens were constructed and tested under reversed cyclic loading. Aspect ratio of the infill wall (l(w)/h(w), where l(w) = infill length, h(w) = infill height) and its placement configurations were the parameters of the experimental study. Test results indicated that partially infilled RC frames exhibited significantly higher ultimate strength and higher initial stiffness than the bare frame (frame with no infill). While the aspect ratio of the infill wall was increased, the lateral strength and the rigidity were increased significantly. In addition to these variables, it has been seen that the connection between frames and infills also affected the behavior of infilled frames. Specimens with partial infill walls both connected to the column and beams of the frame showed the most successful behavior. (c) 2006 Elsevier Ltd. All rights reserved.