São Paulo is the largest city in Brazil and one of the largest cities in the world. In 2004, São Paulo City Department of Health decided to implement a Healthcare Information System to support managing healthcare services and provide an ambulatory health record. The resulting information system is one of the largest public healthcare information systems ever built, with more than 2 million lines of code. Although statistics shows that most software projects fail, and the risks for the São Paulo initiative were enormous, the information system was completed on-time and on-budget. In this paper, we discuss the software engineering principles adopted that allowed to accomplish that project's goals, hoping that sharing the experience of this project will help other healthcare information systems initiatives to succeed.