  The last missing correction to the muon lifetime in the Standard Model at
O(alpha^2) coming from gauge and Higgs boson loops is presented. The associated
contribution to the parameter Delta r in the on-shell scheme ranges from 6
times 10^{-5} to -4 times 10^{-5} for Higgs boson masses from 100 GeV to 1 TeV.
This result translates into a shift of the W boson mass which does not exceed
+/- 1 MeV in the same range and amounts in particular to approximately -0.8 MeV
for a 115 GeV Higgs boson.
