Ferromagnetic full Heusler alloys containing Co are amongst the most studied half-metallic systems. Several studies recently have been concentrated on the effect of defects and impurities. We focus in this Letter on the case of vacancies in these alloys. We show that the occurrence of vacancies at the sites occupied by Co atoms can destroy half-metallicity andalters the Slater-Pauling rule. Such defects are likely to occur since they result to the Cl-b lattice structure of the semi-Heusler alloys. Contrary, the appearance of vacancies at the other sites keeps the half-metallic character of the parent alloys. Thus for realistic devices it is important to prevent the appearance of vacancies during the growth of thin films. (c) 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.