The size distributions of internal pores within three cast model Al alloys have been characterized by X-ray micro tomography. Those distributions have been used as input for a statistical model, coupled to a Paris-like law, for the calculation of fatigue life distribution in cast alloys. The results of the model are in good agreement with the experimental fatigue lives at high stresses (240 MPa). At lower stresses (130 MPa), however, the fatigue lives are not predicted correctly. In that case, a different dislocation based model describing the blocking/unblocking of cracks at grain boundaries is proposed.