  We calculate a two-loop effective potential to the order of {\cal
O}(\lambda_t^2\alpha_s) in the MSSM. We then study the corresponding two-loop
corrections to the CP-even Higgs-boson mass for arbitrary \tan\beta and
left-right top-squark mixings. We find that the lightest Higgs-boson mass is
changed by at most a few GeV. We also show the improved scale dependence and
compare to previous two-loop analyses.
