  We present a scaling study of lattice QCD with O(a) improved Wilson fermions
and a chirally twisted mass term. In order to get precise results with a
moderate computational effort, we have considered a system of physical size of
0.75^3 * 1.5 fm^4 with Schroedinger functional boundary conditions in the
quenched approximation. Looking at meson observables in the pseudoscalar and
vector channels, we find that O(a) improvement is effective and residual cutoff
effects are fairly small.
