  We investigate the ground-state energy of the pi- p atom (pionic hydrogen) in
the framework of QCD+QED. In particular, we evaluate the strong energy-level
shift. We perform the calculation at next-to-leading order in the low-energy
expansion in the framework of the relevant effective field theory. The result
provides a relation between the strong energy shift and the pion-nucleon S-wave
scattering lengths - evaluated in pure QCD - at next-to-leading order in
isospin breaking and in the low-energy expansion. We compare our result with
available model calculations.
