This study explores mortality data as a basis for estimating the irregular migrant population, a technique labelled “mortality extrapolation”. Using Belgian data, it demonstrates that mortality can be a reliable and methodologically sound basis for gauging not only the size of mobile populations, but also their evolution over time, offering granularity in terms of age groups and gender. The technique boosts significant advantages over traditional methods, such as reliance on well-established demographic tools, unbiased observation of basic indicators and a transparent basis for calculating extrapolation factors. Given the type of data that it requires, the method offers promising prospects for broader international implementation.