This paper's primary objective is to contribute to the ‘never-ending story’ of estimating irregular migration. It presents an attempt to develop an empirically based approach/technique for estimating the size of an irregular migrant population, or more specifically of irregularly residing immigrants within the city of Prague. A pilot study, based on discovering the actual number of immigrants residing in selected localities, was conducted via observations and inquiries to local residents, and, subsequently, by comparing these numbers with local official statistics concerning inhabitants. Getting to know the types of built environment in selected localities aided in efforts to extrapolate an estimate for the entire city. Moreover, the spatial distribution of irregular migrants within a city is discussed.