Kinetics of three-dimensional normal grain growth and related processes (e.
g., soap froth evolutions) described by the Mulheran-Harding model is studi
ed. The model is represented by a diffusion equation with the grain-size-de
pendent diffusion coefficient. The equation is solved for an arbitrary init
ial distribution of grain sizes. It is proved that asymptotic kinetics do n
ot depend on the initial state.