  In seesaw models with more than one generation of light and heavy neutrinos,
nu and N, respectively, it is possible to have sizable mixing between them for
heavy-neutrino masses of order 100 GeV or less. We explore this possibility
further, taking into account current experimental constraints, and study its
effect on Higgs-boson decays in the contexts of seesaw models of types I and
III. We find that in the type-I case the Higgs decay into a pair of light and
heavy neutrinos, h -> nu N, could increase the total Higgs width in the
standard model by up to almost 30% for a relatively light Higgs-boson, which
would significantly affect Higgs searches at the LHC. The subsequent prompt
decay of N into three light fermions makes this Higgs decay effectively a
four-body decay. We further find that, in the presence of the large light-heavy
mixing, these four-body Higgs decays can have rates a few times larger than
their standard-model counterparts and therefore could provide a potentially
important window to reveal the underlying seesaw mechanism.
