OBJECTIVE To describe the development of the public health surveillance system for the Sydney 2000 Olympic Games; document its major findings; and discuss the implications for public health surveillance for future events. METHODS Planning for the system took almost three years. Its major components included increased surveillance of communicable diseases; presentations to sentinel emergency departments; medical encounters at Olympic venues; cruise ship surveillance; environmental and food safety inspections; surveillance for bioterrorism; and global epidemic intelligence. A daily report integrated data from all sources. METHODS Sydney, Australia. Surveillance spanned the period 28 August to 4 October 2000. METHODS Residents of Sydney, athletes and officials, Australian and international visitors. RESULTS No outbreaks of communicable diseases were detected. There were around 5% more presentations to Sydney emergency departments than in comparable periods in other years. Several incidents detected through surveillance, including injuries caused by broken glass, and a cluster of presentations related to the use of the drug ecstasy, prompted further action. CONCLUSIONS Key elements in the success of public health surveillance for the Games included its careful planning, its comprehensive coverage of public health issues, and its timely reporting and communication processes. Future systems need to be flexible enough to detect the unexpected.