  A search for the decay KS -> mu+ mu- is performed, based on a data sample of
1.0 fb^-1 of pp collisions at \sqrt{s}=7 TeV collected by the LHCb experiment
at the Large Hadron Collider. The observed number of candidates is consistent
with the background-only hypothesis, yielding an upper limit of BR(KS -> mu+
mu-) < 11 (9) x 10^-9 at 95 (90)% confidence level. This limit is a factor of
thirty below the previous measurement.
