Simulations of Joint European Torus (JET) [P. H. Rebut and et al. , Nucl. Fusion 25 , 1011 (1985)] type I high-mode H-mode discharges with edge localized modes ELMs are used to study the effect of isotope mass on the height of the pedestal and the frequency of ELMs. A dynamic model for the H-mode pedestal and ELM cycles is employed in the JETTO integrated modeling code [M. Erba and et al. , Plasma Phys. Control. Fusion 39 , 261 (1997)]. The stability criteria that are used to trigger ELM crashes in the JETTO simulations are calibrated with the HELENA and MISHKA ideal magnetohydrodynamic MHD stability codes [A. B. Mikhailovskii and et al. , Plasma Phys. Rep. 23 , 713 (1997)]. Results obtained using a pedestal model in which the pedestal width increases with isotope mass are compared with those obtained using a fixed, prescribed pedestal width. In JET type I ELMy H-mode discharges, it is found that the height of the pressure pedestal increases and that the frequency of the ELMs decreases as the isotope mass is increased. Both of these experimentally observed trends are obtained simultaneously in the simulations only if the pedestal width increases with isotope mass. The physical processes that play a significant role in these simulations are described.