Background. Trastuzumab is a monoclonal antibody that together with chemotherapy significantly improves time to progression and overall survival for metastatic breast cancer patients with tumours overexpressing HER2. The aim of this study was to analyse the cost-effectiveness of HER2 testing and trastuzumab in combination with chemotherapy compared with chemotherapy alone from a societal perspective in a Swedish setting. Material and methods. We used a Markov state transition model to simulate HER2 testing and subsequent treatment in a hypothetical cohort of 65 year old metastatic breast cancer patients. Outcomes included life-time costs, quality adjusted life years (QALY), and cost per QALY gained. Five different testing and treatment strategies were evaluated. Results. We estimated the cost per QALY gained to be about 485 000 SEK for the strategy of IHC testing for all patients, with FISH confirmation of 2+ and 3+, and trastuzumab and chemotherapy treatment for FISH positive patients. For the strategy of FISH testing for all patients, with trastuzumab and chemotherapy for FISH positive patients, we estimated the cost per QALY gained to about 561 000 SEK. The remaining testing and treatment strategies were dominated. Results were sensitive to changes in utilities, the risk of breast cancer related death, and test characteristics. Conclusion. Our analysis indicate that FISH testing for all patients with trastuzumab and chemotherapy treatment for FISH positive patients is a cost-effective treatment option from a societal perspective.