  We gereralize the Kochegov-McLerran formula for the ratio
$\sigma^{DD}/\sigma_{tot}$ in perturbative QCD, using Mueller-Glauber approach
for shadowing corrections and AGK cutting rules. We investigate several
phenomenological approaches with the goal of obtaining results consisent with
experimental data. We fail to reproduce the observed weak energy dependence of
the ratio, and conclude that the soft nonperturbative contribution present at
short distances must also be included.
