  We assume that the mass matrix of active neutrinos has hierarchical form with
small mixings, similar to quark mass matrix. We show that the large mixings
between $\nu_\mu$ and $\nu_\tau$ as well as between $\nu_e$ and certain
combination of $\nu_\mu$ and $\nu_\tau$ required by the present data can appear
due to the presence of a sterile neutrino state. Two realizations of this
possibility are considered: (i) Large flavor mixing appears as a result of
sterile neutrino ``decoupling'' ($m_{ss} \gg m_a$), so that the active-sterile
mixings are negligible. (ii) Sterile neutrino has a mass of O(1) eV and its
mixing with active neutrinos can be observable. In the second case, the (3+1)
scheme of neutrino mass, which also accommodates the LSND result, can be
reproduced, provided the hierarchy of the mass matrix of active neutrinos is
not strong (the ratio of largest to smallest elements is about two orders of
magnitude). The enhancement of lepton mixing via coupling with sterile neutrino
can be realized in Grand Unified theories with the see-saw mechanism of
neutrino mass generation.
