  We present a first numerical study of lattice QCD with O(a) improved Wilson
quarks and a chirally twisted mass term. Renormalized correlation functions are
derived from the Schroedinger functional and evaluated in an intermediate
space-time volume of size 0.75^3 x 1.5 fm^4. In the quenched approximation
precise results are then obtained with a moderate computational effort,
allowing for a detailed study of the continuum approach. The latter is
discussed in terms of observables which converge to meson masses and decay
constants in the limit of large space-time volume. In the O(a) improved theory
we find residual cutoff effects to be at the level of a few percent for lattice
spacings of about 0.1 fm.
