State Medical Aid is a public health insurance program that allows undocumented immigrants with low financial resources to access health care services for free. However, the low take-up rate of this program might threaten its efficiency. The purpose of this study is therefore to provide the determinants of such a low take-up rate. To this end, we rely on the Premier Pas survey. This is an original representative sample of undocumented immigrants attending places of assistance to vulnerable populations in France. Determinants of State Medical Aid take-up are analyzed through probit and Cox modeling. The results show that only 51% of those who are eligible for the State Medical Aid program are actually covered, and this proportion is higher among women than among men. The length of stay in France is the most important determinant of take-up. It is worth noting that State Medical Aid take-up is not associated with chronic diseases or functional limitations and is negatively associated with poor mental health. There is, therefore, mixed evidence of health selection into the program. Informational barriers and vulnerabilities experienced by undocumented immigrants are likely to explain this low take-up.