In order to investigate the universal features of collective behavior due to the many‐body interactions, we perform two types of computer simulations on hard‐sphere systems, a Brownian‐dynamics simulation on polydisperse suspensions of hard spheres, where the hydrodymamic interactions between particles are neglected, and a molecular‐dynamics simulation on atomic systems of hard spheres. Thus, we show that the long‐time self‐diffusion coefficient in atomic systems has the same form as that derived theoretically by Tokuyama and Oppenheim (TO) for the monodisperse suspension by taking into account the many‐body hydrodynamic interactions, except that the singular point is now replaced by a new one. We also show that the difference between two coefficients in both systems can be well explained by the short‐time self‐diffusion coefficient derived theoretically for a wide range of volume fractions. © 2004 American Institute of Physics