  We study isospin breaking instanton corrections to the operator product
expansion of the nucleon correlation functions. After a comparison with quark
model calculations based on the 't Hooft interaction, we examine the role of
instantons in the corresponding QCD sum rules. Instanton contributions are
found to be absent in the chirally even sum rule, but significant in the
chirally-odd one. They improve the consistency of both sum rules and favor a
value of the isovector quark condensate close to the chiral estimate.
