  We study thermal leptogenesis induced by decays of a scalar SU(2)_L triplet.
Despite the presence of gauge interactions, unexpected features of the
Boltzmann equations make the efficiency close to maximal in most of the
parameter space. We derive the maximal CP asymmetry in triplet decays, assuming
that it is generated by heavier sources of neutrino masses: in this case
successful leptogenesis needs a triplet heavier than 2.8 10^{10} GeV and does
not further restrict its couplings, allowing detectable mu --> e gamma, tau -->
mu gamma rates in the context of supersymmetric models. Triplet masses down to
the TeV scale are viable in presence of extra sources of CP-violation.
