  We discuss the possible enhancement of the tau lepton events at LHC when the
left-handed stau doublet becomes light (which can be even lighter than the
right-handed stau). This is illustrated in the constrained supersymmetric
seesaw model where the slepton doublet mass is suppressed by the effects of a
large neutrino Yukawa coupling. We study a few representative parameter sets in
the sneutrino coannihilation regions where the tau sneutrino is NLSP and the
stau coannihilation regions where the stau is NLSP both of which yield the
thermal neutralino LSP abundance determined by WMAP.
