We study the Standard Model short-distance prediction for the mass and lifetime differences between the two neutral D meson mass eigenstates. We find that, despite αs/4π suppression, next-to-leading order (NLO) short-distance QCD corrections exceed the corresponding leading order (LO) amplitudes. For the lifetime difference, this stems from the lifting of helicity suppression of a light-quark intermediate state. We find yD is given by yNLO to a reasonable approximation but xD is greatly affected by destructive interference between xLO and xNLO. The net effect is to render yDxD6×10−7. Our NLO short-distance results, still smaller than most long-distance estimates, depend on the same two nonperturbative matrix elements of four-quark operators as in leading order.