This paper deals with the relationship between population density and public transport efficiency. It is focused on mid-size cities with a population of 100 000 inhabitants. The research question is, how strong the relationship is and if increasing residential density can be an effective tool for public transport promotion. The results indicate that increasing residential density could have an effect for public transport use only in neighbourhoods with lower density. Large part of the cities’ population lives in prefabricated housing estates with high population density about 100 inhabitants per hectare. However there is no significantly effect, when the density is increasing above ca. 70 inh./ha. The results could be a contribution to the debate, under what conditions increasing residential density is an effective tool for creating sustainable city.