OBJECTIVE: To determine the cost-effectiveness of a physical activity programme (Be Active) aimed at city-dwelling adults living in Birmingham, UK. METHODS: Very little is known about the cost-effectiveness of public health programmes to improve city-wide physical activity rates. This paper presents a cost-effectiveness analysis that compares a physical activity intervention (Be Active) with no intervention (usual care) using an economic model to quantify the reduction in disease risk over a lifetime. Metabolic equivalent minutes achieved per week, quality-adjusted life years (QALYs) gained and healthcare costs were all included as the main outcome measures in the model. A cost-benefit analysis was also conducted using 'willingness-to-pay' as a measure of value. RESULTS: Under base-case assumptions-that is, assuming that the benefits of increased physical activity are sustained over 5 years, participation in the Be Active programme increased quality-adjusted life expectancy by 0.06 years, at an expected discounted cost of £3552, and thus the cost-effectiveness of Be Active is £400 per QALY. When the start-up costs of the programme are removed from the economic model, the cost-effectiveness is further improved to £16 per QALY. The societal value placed on the Be Active programme was greater than the operation cost therefore the Be Active physical activity intervention results in a net benefit to society. CONCLUSIONS: Participation in Be Active appeared to be cost-effective and cost-beneficial. These results support the use of Be Active as part of a public health programme to improve physical activity levels within the Birmingham-wide population.