  The leading electroweak MSSM two-loop corrections to the electroweak
precision observables are calculated. They are obtained by evaluating the
two-loop O(\alpha_t^2), O(\alpha_t \alpha_b), O(\alpha_b^2) contributions to
the quantity \Delta\rho in the limit of heavy scalar quarks, i.e. we consider
the contributions of a Two-Higgs-Doublet model with MSSM restrictions. The full
analytic result for arbitrary values of the lightest CP-even Higgs boson mass
is presented. The numerical effects of the leading electroweak MSSM two-loop
corrections on the precision observables M_W and sin^2(theta_eff) are analyzed.
The electroweak two-loop contribution to M_W amounts up to -12 MeV and up to +6
10^-5 for sin^2(theta_eff). The corrections from the bottom quark loops can
become important for large values of tan beta. They enter with a different sign
than the O(\alpha_t^2) corrections. We furthermore investigate the current
sensitivity of the electroweak precision observables to the top Yukawa coupling
in the SM and the MSSM. The prospects for indirectly determining this coupling
at the next generation of colliders are discussed.
