  We analyze the azimuthal dependence of the heavy-quark-initiated
contributions to the lepton-nucleon deep inelastic scattering (DIS). First we
derive the relations between the parton level semi-inclusive structure
functions and the helicity $\gamma^{*}Q$ cross sections in the case of
arbitrary values of the heavy quark mass. Then the azimuth-dependent ${\cal
O}(\alpha_{s})$ lepton-quark DIS is calculated in the helicity basis. Finally,
we investigate numerically the properties of the $\cos\phi$ and $\cos2\phi$
distributions caused by the photon-quark scattering (QS) contribution. It turns
out that, contrary to the basic photon-gluon fusion (GF) component, the QS
mechanism is practically $\cos2\phi$-independent. This fact implies that
measurements of the azimuthal distributions in charm leptoproduction could
directly probe the charm density in the proton.
