  We present the pion-pion elastic scattering amplitude at finite temperature
to one-loop in Chiral Perturbation Theory. The thermal scattering amplitude
properly defined allows to generalize the perturbative unitarity relation to
the $T\neq 0$ case. Our result provides a model independent prediction of an
enhanced pion-pion low-energy phase shift with the temperature and it has
physical applications within the context of Relativistic Heavy Ion Collisions.
