  We study the possibility of observing an `invisible' Higgs boson in central
exclusive diffractive production at the LHC. We evaluate the cross section
using, as a simple example, the Standard Model with a heavy fourth generation,
where the invisible decay mode $H \to \nu_4 \bar{\nu}_4$ dominates, with the
heavy neutrino mass $M(\nu_4) \simeq 50$ GeV. We discuss the possible
requirements on trigger conditions and the background processes.
