  We investigate the origin of the cos 2 phi azimuthal asymmetry in unpolarized
semiinclusive DIS. The contributions to this asymmetry arising from the
intrinsic transverse motion of quarks are explicitly evaluated, and predictions
for the HERMES and COMPASS kinematic regimes are presented. We show that the
effect of the leading-twist Boer-Mulders function h_1^perp(x, k_T^2), which
describes a correlation between the transverse momentum and the transverse spin
of quarks, is quite significant and may also account for a part of the cos 2
phi asymmetry measured by ZEUS in the perturbative domain.
