  We have observed the rare decay K+ --> pi+ mu+ mu- and measured the branching
ratio Gamma(K+ --> pi+ mu+ mu-)/Gamma(K+ --> all) = (5.0 +/- 0.4 (stat.) +/-
0.7 (sys.) +/- 0.6 (theor.)) x 10^{-8}. We compare this result with predictions
from chiral perturbation theory and estimates based on the decay K+ --> pi+ e+
e-.
