Ab initio calculations of the electronic structure of yttrium disilicide have been performed using the LMTO-ASA method (linear muffin-tin orbitals in the atomic sphere approximation). Two different cases have been investigated: stoichiometric YSi₂ and silicon-deficient YSi_1.7 where one silicon atom out of every six is missing. The authors show that the main effect of these vacancies is to shift the Fermi level towards a region of much lower density, thus stabilizing the structure.