The level of market concentration varies between industries. According to the findings of many authors, the level of market concentration is affected by many determinants. The previous empirical studies have tried to detect the determinants that have a crucial impact on the level of concentration. The findings of empirical researches are not particularly matched, and in some cases are even contradictory. We think that differences in findings result from methodological differences, and the difference of the data sets, which were analyzed. In this paper our attempt was, based on correlation and regression analysis, to detect the dominant determinants of market concentration. We used the data for US manufacturing industry.