Current neutrino oscillation data strongly suggest that, in the lepton flavor basis, there exist a mu-tau exchange invariance on neutrino mass matrix. Such a symmetry gives a simple explanation of the observable values of atmospheric mixing angle, which seems maximal, and the smallness (null value) of theta_{13} angle in the PMNS mixing matrix. The most general breaking of mu-tau symmetry is parameterized by a sole couple of mass parameters, which encode all possible physical sources. This provides a framework where existing neutrino data is found to constrain the possible values for theta_{13}, strongly depending on neutrino mass hierarchy. Largest allowed values for theta_{13}, reachable by future experiments, are possible only for given cases, thus, pinpointing particular configurations of neutrino mass spectrum, aside of locating the predicted atmospheric angle a sigma deviation away from its current central value.