This paper aims to open a discussion on the relevance of the laboratory studies on heat-induced internal sulphate attack. The results presented here show that specimens of mortars conserved in frequently renewed deionised water expand after 3 years and a half. The same specimens conserved in water that is not renewed still do not expand after more than 5 years of conservation. So, it seems that the appearance of the phenomenon is linked to the experimental conditions of the test. Moreover, the period before the beginning of the expansion is very long. These results raise two important questions: may the laboratory studies be used for the prediction of the field DEF behaviour and, if so, what minimal duration should formulation qualification tests have to ensure the durability of the constructions faced with this pathology?