  The introduction of a chirally twisted mass term has been proposed as an
attractive approach to O(a) improvement of Quantum Chromodynamics with Wilson
fermions on a lattice. For numerical simulation projects it is important to
know the phase structure of the theory in the region of small quark masses. We
study this question in the framework of chiral perturbation theory.
Generalizing the analysis of Sharpe and Singleton we find extensions of normal
and Aoki phase scenarios and a possible new phase with spontaneous breakdown of
chiral symmetry to a discrete $Z_2$.
