BACKGROUND: Large-scale, one-off sporting or physical activity (PA) events are often thought to impact population PA levels. This article reviews the evidence and explores the nature of the effect. METHODS: A search of the published and grey literature was conducted to July 2005 using relevant databases, web sources, and personal contacts. Impacts are described at the individual, societal and community, and environmental levels. RESULTS: Few quality evaluations have been conducted. While mass sporting events appear to influence PA-related infrastructure, there is scant evidence of impact on individual participation at the population level. There is some evidence that events promoting active transport can positively affect PA. CONCLUSIONS: The public health potential of major sporting and PA events is often cited, but evidence for public health benefit is lacking. An evaluation framework is proposed.