We have compared the decay rates of K and K toΠ Π Π Π final states using a subset of the data from the KTeV experiment (E832) at Fermilab. We find that the direct-CP-violation parameter Re (μ’/μ) is equal to [28.0±3.0(stat)±2.8(syst)]×10 . This result definitively establishes the existence of CP violation in a decay process. © 1999 The American Physical Society. L S + - 0 0 -4