  We perform a diagrammatic calculation of the leading two-loop QCD corrections
to the masses of the neutral CP-even Higgs bosons in the Minimal Supersymmetric
Standard Model (MSSM). The results are valid for arbitrary values of the
parameters of the Higgs and scalar top sector of the MSSM. The two-loop
corrections are found to reduce the mass of the lightest Higgs boson
considerably compared to its one-loop value. The numerical results are analyzed
in the GUT favored regions of small and large \tan \beta. Their impact on a
precise prediction for the mass of the lightest Higgs boson is briefly
discussed.
