  We perform a complete calculation of charge symmetry breaking effects for the
reaction pn -> d pi0 at leading order in chiral perturbation theory. A new
leading-order operator is included. From our analysis we extract \delta
m_N^{str}, the strong contribution to the neutron-proton mass difference. The
value obtained, \delta m_N^{str} = 1.5 \pm 0.8 (exp.) \pm 0.5 (th.) MeV, is
consistent with the result based on the Cottingham sum rule. This agreement
provides a non-trivial test of our current understanding of the chiral
structure of QCD.
