In a broad sense, corporate governance refers to a practice system aimed at reducing agency costs derived from conflicts between owners, shareholders and corporate agents. Theoretically, eventual reductions in costs achieved by corporate governance would mean better performance. However, recent studies reveal there is no consensus concerning the effects of governance upon the performance of those companies, showing that its impact on them is a controversial matter. This paper investigates that issue using a sample of twenty shareholding Brazilian companies included in the Bovespa list that implemented governance practice until 2002 and allowed the analysis of their annual balance sheets. The nonparametric Mann-Whitney U Test provided statistics to evaluate whether their performance was significantly different after the implementation mentioned above. In those cases, the relation between governane and performance was controversial, as many indicators did not present statistically meaningful variations after the inclusion of the firms in the Bovespa list as compared to before. One of the possible causes is the fact that the studied companies may have absorbed previously the strong impact of the implementation of governance practices. Results indicate it is not possible to assure that there is a relationship between corporate governance and firm performance.