  The pi+pi- atom decays -in the ground state - predominantly into two neutral
pions. We present a general expression for the corresponding decay width in the
framework of QCD (including photons). It contains all terms at leading and
next-to-leading order in isospin breaking. The result allows one to evaluate
the combination |a_0-a_2| of pi-pi S-wave scattering lengths from pi+pi-
lifetime measurements, like the one presently performed by the DIRAC experiment
at CERN.
