We derive closed expressions and useful expansions for the contributions of the tree-level W-boson propagator to the muon and leptonic decay rates. Calling M and m the masses of the initial and final charged leptons, our results in the limit m ¼ 0 are valid to all orders in M 2 =M 2 W . In the terms of Oðm 2 j =M 2 W Þ (m j ¼ M, m), our leading corrections, of OðM 2 =M 2 W Þ, agree with the canonical value ð3=5ÞM 2 =M 2 W , while the coefficient of our subleading contributions, of Oðm 2 =M 2 W Þ, differs from that reported in the recent literature. A possible explanation of the discrepancy is presented. The numerical effect of the Oðm 2 j =M 2 W Þ 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.The correction of Oðm 2 =M 2 W Þ to the muon decay rate, arising from the tree-level W-boson propagator, is well known in the literature and amounts to a correction factor 1 þ ð3=5Þm 2 =M 2 W . An analogous result was first derived by Lee and Yang in the framework of nonlocal extensions of the Fermi theory [1]. Calling M and m the masses of the initial and final leptons, recent papers have included both the leading corrections, of OðM 2 =M 2 W Þ, as well as the subleading contributions, of Oðm 2 =M 2 W Þ, to the and leptonic decay rates [2][3][4].In the present paper, we evaluate the corrections to the and leptonic decay rates induced by the W-boson propagator in two cases: (i) in the limit m ¼ 0, we derive a closed expression, valid to all orders in M 2 =M 2 W , as well as a useful expansion in powers of M 2 =M 2 W ; (ii) in the corrections of Oðm 2 j =M 2 W Þ (m j ¼ M, m), we evaluate the leading contributions, of OðM 2 =M 2 W Þ, as well as the subleading ones, of Oðm 2 =M 2 W Þ. In the calculation of the latter, it is important to include the contribution of the Àq q =M 2 W term in the unitary-gauge W-boson propagator or, equivalently, in other gauges, that of the associated Goldstone boson. In fact, this term leads to contributions of Oðm 2 =M 2 W Þ. Our result for (ii) is compared with those reported in the recent literature. In the Appendix we present expressions valid for arbitrary values of M W , M, and m in the range M W > M > m.We focus our attention on decay and later on we extend the results to the leptonic decay rates in a straightforward manner. Defining(1) the terms of Oðx n Þ (n ! 1) are very small. For this reason they are evaluated at the tree level, i.e. to zeroth order in .On the other hand, the QED correction to muon decay in the V-A Fermi theory is very important in the term of zeroth order in x. In order to obtain simple expressions, we follow the usual procedure of factorizing out the QED correction ½1 þ in all the terms of order x n (n ! 0) (see, for example, Ref.[5]). This factorization induces terms of Oðx n Þ (n ! 1), which are however extremely small. As a consistency check, we have carried ou...