São Paulo is the largest city in Brazil and one of the largest in the world. In 2004, São Paulo City Department of Health decided to implement a Healthcare Information System that would support managing healthcare services and provide an ambulatory health record. The system was designed to build on from national standards that identify healthcare workers, organizations and the relationships among them and reuse as much as possible existing concepts and software. At the same time, the system should reduce fragmentation, not only by integrating existing redundant or competing systems but also by providing a framework in which new modules would be naturally integrated to each other. Today, the web-based system, known as SIGA Saúde, runs in more than 370 healthcare units, processes some 8 thousand appointment scheduling daily and more than 10 thousand high-cost procedure authorizations monthly. This paper profiles the São Paulo City and its needs and describes the project, obstacles and results so far.