OBJECTIVES: Trastuzumab, a monoclonal antibody, is used in the adjuvant treatment of HER2+ breast cancer. Several different doses and schedules are used, but the optimal duration or dosing frequency of trastuzumab is not known. Most clinical evidence is from 1-year treatment, but shorter 9-week treatment has also been studied. The aim of this study was to evaluate the budget impact of the different treatment protocols. METHODS: The perspective of the study was of a single hospital district with 250,000 inhabitants (i.e. payer), in which all trastuzumab treatment is concentrated to one hospital. Clinical experts described the current treatment, which included both short and long administration of trastuzumab. A budget impact model, which took into account the number of patients, HER2+ prevalence, length and cost of treatment, as well as effectiveness, was created. Estimates concerning distant disease free survival (DDFS) were derived from the published data of FinHer-trial. The effectiveness of both long- and short-term treatment was assumed to be equal. Treatment costs for different stages of breast cancer were indentified. RESULTS: In one year, adjuvant trastuzumab treatment caused a total net budget impact of €119,000 and €619,000 if all patients were treated with either short or prolonged therapy alone, respectively. The difference was around €500,000 in one year. However, in a 4-year period the impact of using long instead of short therapy was €2 million. The annual incidence of breast cancer in the area was 176 and the prevalence of HER2+ was 12%. Adjuvant treatment with trastuzumab prolongs DDFS and thus reduces the need of treatment of advanced breast cancer. CONCLUSIONS: Length of treatment had a substantial influence on costs. Investments in finding optimal treatment strategy for expensive medications may appear highly cost-effective. Future trials will show to what extent the duration of trastuzumab treatment affects the effectiveness of the therapy.