  We compute the dominant two-loop electroweak correction, of O(G_F M_t^2), to
the partial width of the decay of an intermediate-mass Higgs boson into a pair
of photons. We use the asymptotic-expansion technique in order to extract the
leading dependence on the top-quark mass plus four expansion terms that
describe the dependence on the W- and Higgs-boson masses. This correction
reduces the Born result by approximately 2.5%. As a by-product of our analysis,
we also recover the O(G_F M_t^2) correction to the partial width of the
Higgs-boson decay to two gluon jets.
