  Relativistic effects are investigated in nuclear matter calculations
employing renormalized low-momentum nucleon-nucleon ($NN$) interactions. It is
demonstrated that the relativistic effects cure a problem of non-relativistic
low-momentum interactions, which fail to reproduce saturation of nuclear
matter. Including relativistic effects, one already obtains saturation in a
Hartree-Fock calculation. Brueckner-Hartree-Fock calculations lead to a further
improvement of the saturation properties. The results are rather insensitive to
the realistic $NN$ interaction on which they are based.
