In 2013, the Polish enterprise sector generated 73,5% of GDP, of which small and medium-sized enterprises generated 50,1% of GDP – i.e. every second zloty of this value. Among all groups of enterprises according to their sizes, the largest share in generating GDP had microenterprises – approx. 30,8%. The aim of the study was to analyze the degree of changes and tendencies in the development of newly created enterprises – according to the length of years of operation and size classes – micro, small, medium-sized and large enterprises in Poland. The analysis of enterprises was based on CSO data included in the study entitled “Non-financial enterprises established in the years 2012-2016”. The analysis covered enterprises that were established in the years 2012-2016 and actively performed their business activity until the last examined year. The following conclusions were drawn: In the structure of enterprises in the years 2012-2016, microenterprises were dominant. Together with the increase in the number of years of running business, the number and share of small, medium and large enterprises increased at the expense of microenterprises. The survival rate of enterprises established in the years 2012-2016 is the highest for large, medium-sized and small enterprises. In the analyzed period, the highest number of employees was in microenterprises, but the number of employees was constantly fluctuating over the years. The number of people working in small and medium-sized enterprises has constantly increased along with the length of business activity. The highest remunerations were observed in enterprises (successively) – large, medium-sized, small and micro- enterprises. In microenterprises, there is the largest disparity in the amount of salaries.