We present the exact formula for neutrino oscillations, By resorting to rec
ent results of Quantum Field Theory of fermion mixing, we work out the Gree
n's function formalism for mixed neutrinos, This allows us to obtain an add
itional term with different oscillation frequency. The usual quantum mechan
ical Pontecorvo formula is recovered in the relativistic limit. (C) 1999 El
sevier Science B.V. All rights reserved.