  In this paper we propose a method to compute the running top-Yukawa coupling
in supersymmetric models with heavy mass spectrum based on the "running" and
"decoupling" procedure. In order to enable this approach we compute the
two-loop SUSY-QCD radiative corrections required in the decoupling process. The
method has the advantage that large logarithmic corrections are automatically
resummed through the Renormalization Group Equations. As phenomenological
application we study the effects of this approach on the prediction of the
lightest Higgs boson mass at three-loop accuracy. We observe a significant
reduction of the renormalization scale dependence as compared to the direct
method, that is based on the conversion relation between the running and pole
mass for the top quark. The effect of resummation of large logarithmic
contributions consists in an increased prediction for the Higgs boson mass, an
observation in agreement with the previous analyses.
