  Quantum Chromodynamics on a lattice with Wilson fermions and a chirally
twisted mass term for two degenerate quark flavours is considered in the
framework of chiral perturbation theory. The pion masses and decay constants
are calculated in next-to-leading order including terms linear in the lattice
spacing a. We treat both unquenched and partially quenched QCD. We also discuss
the phase structure of twisted mass lattice QCD.
