We study the standard model prediction for the mass difference between the two neutral D meson mass eigenstates, Δm. We derive a dispersion relation based on heavy quark effective theory that relates Am to an integral of the width difference of heavy mesons, ΔΓ, over varying values of the heavy meson mass. Modeling the mD 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 Δm may be comparable in magnitude to ΔΓ in the standard model.