  The two-pion contribution to hadronic vacuum polarization can be extracted
from tau decay data when isospin violating and radiative corrections are taken
into account. When the dominant corrections are applied to the photon-inclusive
decay of the tau into two pions, one obtains a shift (-12.0 +/- 2.6).10^{-10}
for the anomalous magnetic moment of the muon. The shift appears to be too
small to reconcile the determinations of hadronic vacuum polarization from
existing tau and e+ e- data. The reliability of electromagnetic corrections in
the photon-inclusive tau decay is examined.
