Test of the heavy top expansion in the evaluation of M-W and sin(2) theta(lept)(eff)

In order to test the accuracy of the Heavy Top-mass Expansion (HTE) employe d in recent two-loop calculations of M-w and sin(2) theta(eff)(lept), we co nsider their contributions to subtracted quantities of the form (M-w)(sub) = M-w(M-H) - M-w(M-H(o)) where M-H(o) is a reference point. The results are compared with those obtained by a precise numerical evaluation of all the two-loop contributions involving both the Higgs boson and a fermion loop. F or the choice M-H(o) = 65 GeV, and over the large range 65 GeV less than or equal to M-H less than or equal to 1 TeV, we find very small differences b etween the precise and HTE calculations, amounting to \delta M-w\ less than or equal to 0.8 MeV and \delta s(eff)(2)\ less than or equal to 1.2 x 10(- 5). Although corrections involving light fermions are necessary for the con sistency and test of existent calculations, we also discuss the separate co ntributions from the top-bottom isodoublet. In this case, the differences a re larger, although still small, namely \delta M-w\ less than or equal to 1 .9 MeV and \delta s(eff)(2)\ less than or equal to 4.5 x 10(-5).