Adjuvant trastuzumab (T) significantly reduces the risk of progression and death in HER-2 positive high-risk early breast cancer. The differential benefit of T, administered either sequential or concomitant, has been calculated with 2 comparative meta-analyses of randomized trials. We have meta-analyzed sequential and concomitant arms of 6 T adjuvant trials separately and then calculated the pooled hazard ratios (HRs) for disease-free survival (DFS) and overall survival (OS) in both meta-analyses. Primary cardiac event rates have also been meta-analyzed. In the concomitant T meta-analysis, HRs for DFS and OS were 0.62 and 0.68, respectively (P < 0.0001 and <0.00001 for both endpoints). Conversely, in the sequential T meta-analysis, HRs for DFS and OS were, respectively, 0.74 and 0.87, where P is, however, significant only in the first comparison (P < 0.00001 and P = 0.09). Relative risks (RRs) for major cardiac events (severe cardiac hearth failure or death) are 2.44 (P = 0.07) in the concomitant T meta-analysis and 8.35 (P < 0.0001) in the sequential T meta-analysis. Concomitant adjuvant T therapy seems to give a significant and greater benefit than sequential administration in both DFS and OS, and the number of cases of severe cardiotoxicity does not seem to be higher in concomitant administration than in the sequential one.