  We have analyzed the results of reactor and accelerator neutrino oscillation
experiments in a model with mixing of three neutrino fields and a neutrino mass
hierarchy. It is shown that $ \nu_\mu \leftrightarrows \nu_e $ oscillations
with $ 0.6 \le \Delta m^2 \le 100 \, \mathrm{eV}^2 $ and amplitude larger than
$ 2 \times 10^{-3} $ are not compatible with the existing limits on neutrino
oscillations if the non-diagonal elements of the mixing matrix are small. Thus,
if the excess of positron events recently observed in the LSND experiment is
due to $ \bar\nu_\mu \to \bar\nu_e $ oscillations, the mixing in the lepton
sector is basically different from the CKM mixing of quarks.
