The aim of this paperis to present the methodology and the results of the estimation of the number of foreigners staying in Poland. Administrative data sources were used in the research. The authors adopted the capture-recapture method based on log-linear models. As a result, the number of foreigners staying in Poland in 2015 and 2016 has been estimated at around 500,000 persons (95% CI: 369,000–724,000) and around 744,000 persons (601,000–943,000), respectively. The study is the first comprehensive analysis of this kind which aims at estimating the number of foreigners in Poland, and thus fitsin the current of research on hard-to-survey populations. It has to be remembered, however, that capture-recapture method requires close observance of its strict rules in order to be effective, which is also discussed in depth in the paper.