This research paper answers to the question of whether a unified health system could achieve better advances in terms of access and quality of care, comparing the health care systems of Brazil and Argentina. Brazil is the only country in South America in having implemented a unified health system, the Sistema Unico de Saúde, with the aim of improving access and quality of care. Regardless of the scope of health care policies, a unified system seeks to encourage the participation of all health care sectors and all levels of government, unifying efforts to improve access and quality. Trends in health care quality indicators showed a significant improvement in indicators of access (study I), and quality of care (study II) in Brazil in comparison with Argentina. The implementation of a unified health care system by the reform of 1989 of Brazil’s Constitution acted as a catalyst for the design and implementation of policies, expanding access to health services, as well as improving the quality of care. The positive changes observed in Brazil compared to Argentina could be attributed to the impact of the policies that consolidated Brazil’s Unified Health System in 2001, which would not have been possible without the universal care scope and the unified structure of its health care system initiated by the constitution reform. Results of trend analyses, and multivariate and difference-in-difference regressions on quality indicators suggest that the consolidation of a unified health system had positive impacts on improving and maintaining the quality of care in Brazil, namely in matters of out-of-pocket expenditure, and the mortality from of communicable and noncommunicable diseases. Throughout this study, I extend prior research to provide South American countries with information and insights for policy reform following the example of Brazil’s unified health care system.