  The origin of Lam-Tung relation for the angular asymmetry of Drell-Yan
dileptons is analyzed. The asymmetry constrained by this relation is shown to
belongto the class of kinematic azimuthal asymmetries, emerging due to the
deviation of the reference axis from the natural physical one. The validity and
violation of Lam-Tung relation due to radiative and power QCD corrections is
discussed. The relation is generalized to the case when the virtual photon is
not the transverse oneand the possible applications are suggested.
