This dissertation aimed to verify whether large shareholders - blockholders, not belonging to the control block, in a scenario of low investor protection and high concentration of capital, such as the Brazilian, act as a corporate governance mechanism, reducing agency costs due to the monitoring of managers / controllers. The database comprised non-financial publicly traded companies in Brazil, from 2010 to 2019, totaling 1,091 company-years. Fixed effects regression was used for panel data, and, as a robustness analysis, the Propensity Score Matching, resulting in a negative impact on Tobin s Q for companies that have blockholders in their structure. This result, despite diverging from the international literature, is echoed in other works carried out in the Brazilian scenario, and is justified by the high shareholding concentration of the companies, as blockholders would not represent a real threat to controller s, which generally has more than 50 percent voting capital, due to the low quality of investor protection laws and cultural factors, which encourage the passivity of Brazilian shareholders. The existence of blockholders in this scenario demonstrates a dilution of share capital and cash flow that increases the incentives that managers / controllers have to expropriate the company. Regarding the heterogeneity of the blockholders, it was found that companies with family-type blockholders have Tobin s Q lower than the other types. This result is explained by the fact that family-type blockholders would be more likely to form alliances with the controlling shareholder to expropriate other minority shareholders.