  Data on Ke4 decays allow one to extract experimental information on the
elastic pi pi scattering amplitude near threshold, and to confront the outcome
of the analysis with predictions made in the framework of QCD. These
predictions concern an isospin symmetric world, while experiments are carried
out in the real world, where isospin breaking effects - generated by
electromagnetic interactions and by the mass difference of the up and down
quarks - are always present. We discuss the corrections required to account for
these, so that a meaningful comparison with the predictions becomes possible.
In particular, we note that there is a spectacular isospin breaking effect in
Ke4 decays. Once it is taken into account, the previous discrepancy between
NA48/2 data on Ke4 decays and the prediction of pi pi scattering lengths
disappears.
