BACKGROUND: Large-scale policy and environmental changes, such as congestion road taxes, may be a way to promote active transportation. PURPOSE: This study aimed to examine the potential effect of a congestion road tax on physical activity. METHODS: Baseline data were collected during October-November 2003, follow-up data in May 2006, and analysis was performed in September 2008. The short self-administered version of the International Physical Activity Questionnaire was used to assess physical activity. Data from those with access to motorized vehicles in the Stockholm region (n=165), where the tax was in place, were compared with those from the Goteborg/Malmo regions (n=138). Within each region before and during the road tax implementation, the data were analyzed for differences in time spent at different intensity levels of physical activity, in addition to sitting, as well as for changes in reported time in overall (weighted) physical activity. RESULTS: There were no significant differences in the magnitude of the changes of the intensity levels of physical activity, weighted overall physical activity, or sitting, between Stockholm and Goteborg/Malmo. Among those exposed to the congestion road tax and with access to motorized vehicles, an increase in moderate physical activity (p=0.036); overall physical activity (p=0.015); and a reduction in time spent sitting (p=0.009) was observed. No differences were observed among those unexposed. CONCLUSIONS: The results from this study on the influence of a congestion road tax on levels of physical activity, though inconclusive, suggest that policy changes such as a congestion road tax might promote improvements in physical activity levels in individuals with motorized vehicles.