  We study the Standard Model prediction for the mass difference between the
two neutral D meson mass eigenstates, $\Delta m$. We derive a dispersion
relation based on heavy quark effective theory that relates $\Delta m$ to an
integral of the width difference of heavy mesons, $\Delta\Gamma$, over varying
values of the heavy meson mass. Modeling the $m_D$-dependence of certain D
decay partial widths, we investigate the effects of SU(3) breaking from phase
space on the mass difference. We find that $\Delta m$ may be comparable in
magnitude to $\Delta\Gamma$ in the Standard Model.
