Although corporate governance is often highlighted as a barrier to universalizing sanitation services in Brazil, empirical studies on the subject are scarce. To fill this gap, this study develops a Corporate Governance Index (CGI) for a sample of 53 water and sewage service providers in Brazil. This indicator is used to empirically assess its effects on the technical efficiency of companies in the sector, estimated through data envelopment analysis, from 2015 to 2019. The results of applying linear regression models with different specifications suggest that corporate governance does not significantly affect the technical efficiency of such companies, ceteris paribus. Size, however, seems to be a strong determinant of efficiency. These results directly affect public and private managers and contribute to the ongoing debate on the modernization and improvement of sanitation companies' efficiency in Brazil.