  We compute the O(alpha_t^2) two-loop corrections to the neutral CP-even Higgs
boson mass matrix in the Minimal Supersymmetric Standard Model, for arbitrary
values of mA and of the parameters in the stop sector, in the effective
potential approach. In a large region of parameter space these corrections are
sizeable, increasing the prediction for mh by several GeV. We present explicit
analytical formulae for a simplified case. We discuss the inclusion of
momentum-dependent corrections and some possible ways of assigning the input
parameters.
