  We present a first scaling test of twisted mass QCD with pure Wilson quarks
for a twisting angle of pi/2. We have computed the vector meson mass and the
pseudoscalar decay constant for different values of beta at fixed value of r_0
m_PS. The results obtained in the quenched approximation are compared with data
for pure Wilson and non-perturbatively O(a) improved Wilson computations. We
show that our results from Wilson twisted mass QCD show clearly reduced lattice
spacing errors, consistent with O(a) improvement and without the need of any
improvement terms added. These results thus provide numerical evidence of the
prediction in ref. [1].
