The starting hypothesis is that the market structure to a large extent determines the conduct and the level of the achieved performance of economic entities. The aim of this paper is to determine the theoretical and empirical correlation between market structure, conduct, and performance of economic entities. The paper provides an analytical overview of the mobile telecommunications market in the Republic of Serbia. By using different indicators (the Concentration ratio, the HHI, the Lorenz curve, and the Gini coefficient), the supply concentration in this market is measured and the nature of competition is analysed. By using statistical methods (correlation and simple linear regression analysis), the correlation between the variables, which confirmed the existence of a strong positive correlation between the degree of supply concentration on the market and the level of achieved performance is examined.