Abstract : Estimates of isospin mixing in proton-rich medium-heavy nuclei in the region of mass numbers A=80 and 100 are made with the use of Hartree-Fock (HF) calculations with Skyrme forces. An isospin mixing probability of 4-5 % is predicted in the ground state of 100Sn and about 3-4 % in 80Zr. These are about a factor of 2 larger than the estimates given by Bohr and Mottelson who directly evaluate the isospin mixing with a collective model description of the isovector giant monopole resonance (IVGMR). We show that if we use the energy-weighted sum rule together with the energies of the IVGMR given by Bohr and Mottelson, the predicted isospin admixtures are close to those of HF calculations. More realistic estimates of isospin mixing with random-phase approximation are also presented in the case of 100Sn.