  We present a new method to evaluate with high precision the isospin breaking
effects due to the 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 $m_d - m_u$. In this talk we
discuss how to apply this method to compute the leading isospin breaking
effects for several physical quantities of interest: the kaon masses, the kaon
decay constants and the neutron-proton mass splitting.
