This paper discusses the flexible foundation effects on dynamic responses of engine-mount systems using computer simulation techniques. Equations of motion for the engine-mount systems including flexible foundations are derived. The dynamic flexibility of the foundation is represented by modal information from finite element analysis or experimental modal analysis. Solving the derived equations, natural frequencies and forced vibration responses of an engine-mount system can be simulated accurately. It is shown that the flexibility of the supporting structure may have a significant effect on the idle shake vibration and mounting forces transmitted from the engine to the structure. The computational method developed is applied to an example engine-mount system and the results are compared to those of an associated experiment.