Large samples of hyperon and kaon decays have been collected with 
the HyperCP experiment during two fixed target runs at Fermilab. 
Based on the analysis of 110 million K+/- decays of the 1997 data 
sample we present the branching ratio for K+/- -> pi+/- mu+ mu-. 
This is the first observation of K- -> pi- mu+ mu- decay.