Cast Al-Si alloys have been widely used in automotive applications with regard to their low density and excellent thermal conductivity. Many components made of these alloys are subjected to cyclic loads which can lead to fatigue failure. For these materials, the well-known size effect in fatigue, whereby the fatigue strength is reduced when the size is increased, can be significant and needs to be properly evaluated. This paper analyses the role of casting defects on the fatigue scatter and size effect. A uniaxial fatigue testing campaign (R=0.1) has been conducted using two cast aluminum alloys, fabricated by different casting processes (gravity die casting and lost foam casting), associated with the T7 heat treatment, and with different degrees of porosity. The fatigue response of different specimens (smooth and notched) with different stressed volumes has been investigated. The first part of this article is about the experimental characterization of the size effect and scatter in both alloys via the concept of the Highly Stressed Volume. The second part investigates the effect of the Highly Stressed Volume on the critical defect size and the establishment of Kitagawa-Takahashi diagram. It is shown that the alloy B, with a population of defects of large size, shows a slight size effect and low scatter. In comparison, alloy A that exhibits a population of defects of relatively small size manifests significant size effect and high scatter.< Réduire