  We point out that the recent data from the Sudbury Neutrino Observatory,
together with other relevant measurements from solar and reactor neutrino
experiments, convincingly show that the flavor transitions of solar neutrinos
are affected by Mikheyev-Smirnov-Wolfenstein (MSW) effects. More precisely, one
can safely reject the null hypothesis of no MSW interaction energy in matter,
despite the fact that the interaction amplitude (formally treated as a free
parameter) is still weakly constrained by the current phenomenology. Such a
constraint can be improved, however, by future data from the KamLAND
experiment. In the standard MSW case, we also perform an updated analysis of
two-family active oscillations of solar and reactor neutrinos.
