  We compute two-point functions of chiral operators Tr(\Phi^k) for any k, in
{\cal N}=4 supersymmetric SU(N) Yang-Mills theory. We find that up to the order
g^4 the perturbative corrections to the correlators vanish for all N. The
cancellation occurs in a highly non trivial way, due to a complicated interplay
between planar and non planar diagrams. In complete generality we show that
this same result is valid for any simple gauge group. Contact term
contributions signal the presence of ultraviolet divergences. They are
arbitrary at the tree level, but the absence of perturbative renormalization in
the non singular part of the correlators allows to compute them unambiguously
at higher orders. In the spirit of the AdS/CFT correspondence we comment on
their relation to infrared singularities in the supergravity sector.
