The article presents the spatial aspect of population research in Lithuania, its causes and possibilities for such research. It states that population research in Lithuania is not carried out on a sufficiently detailed scale, so this increases the probability that the emerging or long-standing social problems will not be observed. These problems are especially relevant in cities that have a high diversity of different ethnic, socio-demographic, socio-economic and other population groups. And the main reason why many large-scale population researches have not been carried out in Lithuania is the country’s weak traditions of detailed research and social statistics. There are also a lot of methodological problems with official detailed statistical data submission, which makes difficult to use them. This article proposes the methodology for population research using the detailed population census data of 2011, illustrated by an example of the analysis of the ethnic structure of the three major Lithuanian cities. After the analysis it can be stated that such territorial distribution of the ethnic structure in cities was influenced by historical conditions.