Empirical expressions for the fundamental natural periods of reinforced concrete (RC) moment resisting frame (MRF) buildings with unreinforced masonry (URM) infill walls are developed using data from ambient vibration surveys of 19 such buildings. These improved estimates can be used in equivalent seismic lateral force procedure for seismic design of such buildings. The fundamental natural periods are also obtained from analytical modelling. Comparison of experimental and analytical values with those projected by expressions in Indian seismic code shows a need for revising the relevant Indian code provisions. Also, this study shows that the current methods of modelling RC MRF with URM walls are highly inaccurate in predicting the fundamental natural periods and the mode shapes.