OBJECTIVES: To estimate cost-effectiveness of adjuvant trastuzumab for one year following standard chemotherapy in HER2-positive early breast cancer (EBC) versus standard therapy alone (observation). METHODS: Markov model was developed based on results of pivotal clinical study (HERA). The health states included disease free survival, recurrence (local, contralateral), distant metastases, cardiac adverse events and death. The cohort simulation starts with 50-year-old patients who are considered disease free, i.e. have undergone surgery, chemo- and/or radiotherapy. Effectiveness was measured in terms of QALY gained. Population-based utilities were derived from published literature. Yearly costs of each health state were established based on the opinion of Polish experts from two oncology centers and reported in PLN (1 EUR=3.8 PLN in 2006). Direct medical costs and outcomes were projected over patients lifetimes from a Polish health-care payer's perspective, using 5% discount rate for both or costs only. Probabilistic sensitivity analysis was performed. RESULTS: The total lifetime costs/patient were estimated to be 198,294 PLN in trastuzumab and 62,812 PLN in no-trastuzumab arm respectively. The main cost driver was the aquisition cost of trastuzumab. The total QALYs generated with trastuzumab treatment were 9.53 vs 7.5 for no trastuzumab treatment (discounted) and 16.56 vs 11.9 QALY (without discounting) respectively. This results in ICER for trastuzumab of 66,746 PLN/QALY (discounted) and 30,769 PLN/QALY (with only costs discounted). Probabilistic sensitivity analysis showed, that for 50% simulations the ICER remained below 60,000 PLN/QALY and for 97.5% - below 88,624 PLN/QALY. CONCLUSION: The use of trastuzumab in EBC improves QALYs compared to observation alone. Considering the suggested threshold for cost-effectiveness, calculated on the basis of one-year haemodialysis treatment costs (60,000 – 74,700 PLN), adjuvant therapy with trastuzumab appears to be cost-effective treatment for patients with HER2-positive EBC in Poland.