An elasto-dynamic model for pile-soil-pile interaction together with a simple plate model is used in this study to assess the effect of flexible foundation slabs on the dynamic response of pile groups. To this end, different pile configurations with various slab thicknessesare considered in two soil media with low and high elastic moduli. The analyses include dynamic impedances and seismic responses of pile-group foundations. The presented results indicate that the stiffness and damping of pile foundations increase with thickness of the foundation slab; however, the results approach those for rigid slab as the slab thickness approaches twice the pile diameter for the cases considered in this study. The results also reveal that pile foundations with flexible slabs may amplify the earthquake motions by as much as 10 percent in the low to intermediate frequency ranges.