  We present a new method to evaluate with high precision isospin breaking
effects due to the small mass difference between the up and down quarks using
lattice QCD. Our proposal is applicable in principle to any hadronic observable
which can be computed on the lattice. It is based on the expansion of the
path-integral in powers of the small parameter md-mu. In this paper, we apply
this method to compute the leading isospin breaking effects for several
physical quantities of interest: the kaon meson masses, the kaon decay
constant, the form factors of semileptonic Kl3 decays and the neutron-proton
mass splitting.
