  We derive closed expressions and useful expansions for the contributions of
the tree-level W-boson propagator to the the muon and tau leptonic decay rates.
Calling M and m the masses of the initial and final charged leptons, our
results in the limit m=0 are valid to all orders in M^2/M_W^2. In the terms of
O(m_j^2/M_W^2) (m_j=M,m), our leading corrections, of O(M^2/M_W^2), agree with
the canonical value (3/5) M^2/M_W^2, while the coefficient of our subleading
contributions, of O(m^2/M_W^2), differs from that reported in the recent
literature. A possible explanation of the discrepancy is presented. The
numerical effect of the O(m_j^2/M_W^2) corrections is briefly discussed. A
general expression, valid for arbitrary values of M_W, M and m in the range
M_W>M>m, is given in the Appendix. The paper also contains a review of the
traditional definition and evaluation of the Fermi constant.

High Energy Physics - Phenomenology