The Author discussed, explored, evaluated and diagnosed the disparities of economic activity on regional level. The study embraced five peripheral regions of Poland, namely five Eastern provinces (voivodships), i.e. Lublin Province, Podlasie Province, Subcarpathia Province, Świętokrzyskie Province and Warmia-Masuria Province. The aforementioned five voivodships were compared to Poland and Poland’s central region, i.e. Masovia Province. The following elements were taken into consideration: employment rate and employment by sectors, unemployment rate, GDP per capita, number of economic entities and number of natural persons conducting economic activity, investment outlays and gross expenditure on R&D activity, as well as net internal and external migration for permanent residence. In order to observe the possible changes over the time the situation in 2005 and 2017 was studied. Additionally, the total value of projects co-financed by the European Union through Structural Funds and Cohesion Fund within 2007–2013 period and 2014–2020 perspective was presented and analyzed. The research tools used in the article included literature studies, critical thinking, descriptive analysis and comparative analysis, in that the selected statistical method of multivariate comparative analysis, i.e. Hellwig’s taxonomic measure of development. Statistical material provided by Central Statistical Office (Warsaw, Poland) and taken from Statistical Yearbooks of the Regions 2006, 2007, 2017 and 2018 was used for the analysis. The obtained results indicate the persistence of huge regional disparities of economic activity, economic potential and socioeconomic development despite the implementation of EU Policy of Social, Economic and Territorial Cohesion. Hence, it was possible to positively verify the research hypotheses: both the first hypothesis assuming that the gap between the studied peripheral regions and the rest of Poland was not reduced and the second hypothesis assuming that the participation in the processes of regional economic integration, and in particular in EU Policy of Economic, Social and Territorial Cohesion, had only limited impact on the reduction of disparities between Poland’s regions.