The sand casting process usually generates porosity that is highly detrimental especially to the fatigue behavior of Al-Si alloys. Since pores favor early fatigue crack initiation, the total fatigue life is crack-propagation-dominated and, for a given stress level, strongly depends on the initiating pore size. Here long life fatigue strengths were estimated using specimens extracted from different AlSi7Mg castings. Detailed metallographic characterization of specimen sections was carried out to determine pore size distributions and correlate it to test results. Murakami’s experimental procedure for applying the largest extreme value statistics to small defect characterization is applied and discussed.