Theoretical aspects of the (π ±, π o) reaction in complex nuclei are studied using the impulse approximation and the Chew-Low amplitude for pion-nucleon scattering. Sum rules are developed for the differential cross section integrated over the final pion energy. It is shown that this quantity is sensitive in particular to proton-neutron correlations in the nucleus and thus its measurement may provide useful information on one aspect of nucleon-nucleon correlations. Expressions are presented for the energy-integrated cross section for closed shell or closed subshell nuclei. Numerical results are given in the case of 16O, where it appears that correlations due to the Pauli exclusion principle are far more significant than those due to the nucleon hard core.