Magnetospheric cavity modes can decay through various loss processes. Energy leakage across the magnetopause is one such process that is commonly neglected. Here we consider what the actual effect of the neglected magnetopause leakage might be on cavity mode lifetime and hence on coupled transverse mode wave growth. First, we review the linear theory of fast mode wave reflection at a tangential discontinuity to show that magnetopause leakage is significant. For realistic subsolar magnetopause conditions we estimate that 20--60% of magnetospheric fast mode wave Poynting flux incident on the magnetopause is transmitted through the magnetopause and out of the magnetospheric cavity. Second, we demonstrate the effect of magnetopause leakage on magnetospheric cavity modes using an MHD simulation. The simulation is of a box-like cavity with a dipolar variation of the magnetospheric magnetic field and realistic conditions at the subsolar magnetopause, based upon in situ spacecraft observations. The simulation results show that a purely fast mode cavity oscillation decays due to leakage of fast mode wave energy across the magnetopause. The decay rate that arises from magnetopause leakage is comparable to that found in previous simulations from coupling of fast mode energy into transverse mode field line resonances. Thus we conclude that magnetopause leakage is likely to enhance the decay rate of fast and transverse cavity mode oscillations and should be taken into account in field line resonance theory and especially its application to observations. ¿ 2000 American Geophysical Union