  We extend chiral perturbation theory to include linear dependence on the
lattice spacing $a$ for the Wilson action. The perturbation theory is written
as a double expansion in the small quark mass $m_q$ and lattice spacing $a$. We
present formulae for the mass and decay constant of a flavor-non-singlet meson
in this scheme to order $a$ and $m_q^2$. The extension to the partially
quenched theory is also described.
