The article presents the changes in the urban population density in the 20th c., in central Poland, in an area where the two largest Polish cities – Warsaw and Lodz are situated, within a distance of 120 km from each other. The analysis was conducted by means of a statistical method used for estimating discret distributions – the kernel function method. The conclusions served the purpose of presenting the changes which took place around these cities, as well as an academic discussion regarding the condition and the future of the duopolis in the light of the discussed factor
