Background: One year of adjuvant trastuzumab in combination with chemotherapy is the standard of care in early-stage HER2 positive breast cancer. Existing data on shortening trastuzumab treatment show conflicting results. Methods: A search of PubMed and conferences identified randomized trials that compared abbreviated trastuzumab therapy to one year of treatment in early-stage HER2 positive breast cancer. Hazard ratios (HRs) and 95% confidence intervals (CI) were extracted for disease free survival (DFS) and overall survival (OS). Data on the number of DFS and distant relapse events were also collected as were the number of patients at risk in each group. Subgroup analyses evaluated the effect of nodal involvement, estrogen receptor (ER) expression and the duration of abbreviated trastuzumab (9-12 weeks versus 6 months). Odds ratios (ORs) and 95% CI were computed for pre-specified cardiotoxicity events including cardiac dysfunction and congestive heart failure (CHF). Results: Analysis included 6 trials comprising 11603 patients. In most studies adjuvant chemotherapy included anthracyclines and taxanes. Shorter trastuzumab treatment was associated with worse DFS (HR = 1.14, 95% CI 1.05-1.25, p = 0.002) and OS (HR = 1.15, 95% CI 1.02-1.29. p = 0.02). The effect on DFS was not influenced by ER status (p for the subgroup difference = 0.23), nodal involvement (p = 0.44) or the different duration of trastuzumab in the experimental arm (p = 0.08). In absolute terms, after an estimated median follow-up of 71 months, shorter treatment with trastuzumab was associated with an absolute increase in DFS events of 2.3%. Shorter trastuzumab treatment was associated with lower odds of cardiac dysfunction (OR = 0.67, 95% CI 0.55-0.81, p < 0.001) and CHF (OR = 0.66, 95% CI 0.50-0.86, p = 0.003). Conclusions: Compared to one year, shorter duration of adjuvant trastuzumab is associated with significantly worse DFS and OS, despite favorable cardiotoxicity profile. One year of trastuzumab should remain the standard adjuvant treatment in early-stage HER2 positive breast cancer with appropriate cardiac monitoring.