The main aim of this paper was to assess the factors of farmland abandonment in selected metropolitan areas in Poland. The research used secondary research material, including data from the Main Statistical Office (Polish GUS) and academic literature. Analyses were conducted by means of the method of regression trees, among other things. The research found out that nearly 16% of farmland in Polish metropolitan areas had been abandoned. The factor that most affected set-aside was the share of small farms with an area of less than 5 ha of agricultural land. In communes with the majority of small farms, almost 30% of agricultural land was set aside. Entrepreneurship indicator, population density and net migration were also significant in explaining the phenomenon discussed in the paper. High values of these measures correlated with more advanced processes of farmland abandonment.