  We computed the B_K parameter on the lattice by using a non-perturbatively
improved Wilson action. From our quenched simulation, in the MSbar scheme and
at mu=2 GeV, we obtain B_K=0.73(7)^{+0.05}_{-0.01}. To gain some insight in the
systematic errors due to the use of the quenched approximation, we also made
the unquenched computation with Wilson fermions. We do not observe any
significant deviation with respect to the quenched result.
