Background: Adjuvant trastuzumab in combination with chemotherapy improves outcome of women with HER2 positive early breast cancer. However, the optimal duration of treatment remains unknown. In this study we compared 6 versus 12 months of adjuvant trastuzumab. Methods: Axillary node positive or high risk node negative women with HER2 overexpressing or amplified early breast cancer were randomized following surgery to receive either 6 (arm A) or 12 (arm B) months of adjuvant trastuzumab in combination with dose dense G-CSF-supported Docetaxel (75mg/m² every 14 days for 4 cycles) following FEC (5FU 700mg/m², epirubicin 75mg/m², cyclophosphamide 700mg/m2 every 14 days for 4 cycles). The primary endpoint of the study was the 3-year disease-free survival (DFS). Results: Four hundred eighty one patients were randomized; 240 on arm A and 241 on arm B. Of them 83 (34%) and 100 (41%) were premenopausal, 200 (83%) and 180 (75%) were node positive, 165 (69%) and 156 (65%) were hormone receptor positive in arm A and B, respectively. Chemotherapy was completed in 98% and 99% of patients while trastuzumab therapy in 96% and 100% of patients in arm A and B, respectively. After a median follow up of 43.5 and 42 months there were 26 (10.8%) and 15 (6.2%) (p=0.07) disease relapses and the median DFS has not yet been reached (p=0.08) while the 3-year DFS rate was 92.4% and 95.1% for arm A and B, respectively. Conclusions: Preliminary results of this study in terms of disease relapse and DFS are in favor of 12 months of adjuvant trastuzumab administration. Clinical trial information: NCT00615602.