Migrants without residency permit, known as undocumented, tend to live in precarious conditions and be exposed to an accumulation of adverse determinants of health. Only scarce evidence exists on the social, economic and living conditions-related factors influencing their health status and well-being. No study has assessed the impact of legal status regularisation. The Parchemins study is the first prospective, mixed-methods study aiming at measuring the impact on health and well-being of a regularisation policy on undocumented migrants in Europe.