  We discuss the role of the U(1) axial symmetry for the phase structure of QCD
at finite temperature. In particular, supported by recent lattice results, we
analyse a scenario in which a U(1)-breaking condensate survives across the
chiral transition. This scenario can be consistently reproduced using an
effective Lagrangian model. The effects of the U(1) chiral condensate on the
slope of the topological susceptibility in the full theory with quarks are
studied. Further information on the new U(1) chiral order parameter is derived
from the study (at zero temperature) of the radiative decays of the light
pseudoscalar mesons in two photons.
