  We demonstrate that perturbative QCD leads to positive 3D parton--parton
correlations inside nucleon explaining a factor two enhancement of the cross
section of multi-parton interactions observed at Tevatron at $x_i\ge 0.01$ as
compared to the predictions of the independent parton approximation. We also
find that though perturbative correlations decrease with $x$ decreasing, the
nonperturbative mechanism kicks in and should generate correlation which, at
$x$ below $10^{-3}$, is comparable in magnitude with the perturbative one for
$x\sim 0.01$.
