OBJECTIVES: Britain has one of the highest breast cancer rates in the world with around 36,000 women newly diagnosed each year. With improving survival rates on average 90% of women are still alive five years later. The ATAC trial (median follow-up 47.2 months) confirmed that anastrozole resulted in a 18% reduction in the risk of disease recurrence relative to tamoxifen in this population. Our study aimed to identify the budget impact of anastrozole compared to tamoxifen in postmenopausal women with early breast cancer based on alternative scenarios of uptake over 3 years from the NHS perspective. METHODS: The budget impact model was based on a modelled cost-effectiveness analysis of the ATAC trial data. Published UK data was used to estimate the treatment eligible population each year. Different scenarios about uptake were defined and the net budgetary effects calculated. Costs were discounted at 6% annually. Probabilistic sensitivity analysis was undertaken. RESULTS: For a total number of around 13,200 HR+EBC patients each year the cost of drug treatment with tamoxifen is estimated to be £64.6 million. Under the projected likely scenario of uptake reaching 35% by 2006, the net present value of the incremental drug costs with anastrozole will amount to £18.4 million. This is offset by £3.4 million by avoiding breast cancer recurrences, AEs and follow-up costs. The model is sensitive to the rate of uptake. CONCLUSIONS: The budgetary impact of anastrozole for all available patients is less than 7% of the annual amount spent on breast cancer in the UK. If the subpopulation with high risk of thromboembolic and cardiovascular disease were included the impact will be lower. Other technologies with similar budget impact have been approved but NICE. Accompanying the cost-effectiveness analysis the budget impact is going to be an important input into the forthcoming policy decision about the adoption of anastrozole in EBC patients.