OBJECTIVES: Trastuzumab was approved for the treatment of HER2-positive early breast cancer (EBC) in 2006 in several European countries, Portugal included. The model assesses the impact of trastuzumab treatment in EBC on the annual number of patients entering the metastatic setting between 2005 and 2015 in Portugal. METHODS: Annual EBC incidence for 2005-2015 was projected by applying stage-specific proportions for stages I-III to female breast cancer incidence rates estimated for 2008 (GLOBOCAN 2010), using a conjoint estimate based on English cancer registries, where overall breast cancer survival is comparable to that seen in Portugal. Age-specific rates were applied to UN population projections for 2000-2025. The annual number of patients with HER2-positive metastatic breast cancer (MBC) includes de novo and recurrent MBC patients. The baseline 10-year recurrence rate was estimated as 37%, based on 4-year follow-up in the control arm of a combined trial analysis in patients with HER2-positive breast cancer and the long-term timing of recurrence in all patients with breast cancer. To model recurrence in trastuzumab-treated EBC patients, the hazard ratio (HR) from the HERA trial (0.49; 95% CI: 0.38, 0.63) was applied. RESULTS: In 2004, prior to approval of trastuzumab in EBC, the pool of de novo and relapsed MBC patients was estimated to be 650. Between 2005 and 2015, the model predicts that the use of trastuzumab will result in an average annual decline in recurrence of MBC of 5.2% (95% CI: 3.0, 6.7). Cumulative recurrences prevented by trastuzumab treatment between 2005 and 2014 (projected through full 10-year window for recurrence) is projected to be 884 (95% CI: 537; 1,075). CONCLUSIONS: Trastuzumab treatment of HER2-positive EBC over a 10-year period is expected to prevent nearly 900 women from developing metastases in Portugal, which may result in a similar number of breast cancer deaths being avoided.