Objective: There have been numerous studies on cost-effectiveness of trastuzumab in both treatments of adjuvant and metastatic breast cancer (BC). Nevertheless, the results reported were varied depending upon the assumptions and/or perspectives of the studies. We performed a systematic review of cost-effectiveness-analysis (CEA) studies of trastuzumab in treatment of HER2-postive breast cancer. Methods: Literature search from 1996 to December 2007 on databases including PubMed, Ovid MEDLINE, and HealthSTAR was performed to retrieve CEA studies of trastuzumab, using MESH terms and keywords such as “trastuzumab,” “costs and cost analysis,” “economics,” “breast neoplasm,” “cost effectiveness,” “cost utility,” and “breast cancer.” Additionally, abstracts on CEA studies were also obtained from American Society of Clinical Oncology (ASCO) and ISPOR annual meetings. Only CEA studies reported incremental cost-effectiveness ratio (ICER) or cost-utility ratio (ICUR) as cost per quality-adjusted life years were included in this review. Results: Thirty five studies (20 published articles and 15 abstracts) were identified, of which 18 studies (14 adjuvant, 3 metastatic BC studies, and 1 study of product life-cycle of trastuzumab) representing societal healthcare perspectives from 12 countries were satisfied the criteria. The mean (median) ICERs of trastuzumab are $24,069/QALY ($23,766/QALY) [ranged from $4,767 to $58,414/QALY] and $88,373/QALY ($80,000/QALY) [ranged from $60,120 to $125,000/QALY] for HER2-postive adjuvant and metastatic breast cancer treatments, respectively. Majority of sensitivity analyses showed the main cost driver was the acquisition cost of trastuzumab. In addition, over the product life-cycle of trastuzumab, the overall ICER is $34,400/QALY (Garrison et al., 2006). Conclusion: This review suggests that the average costs per QALY of trastuzumab in both treatments of adjuvant and metastatic HER2-positive breast cancer are consistent and below the suggested cost effectiveness threshold of $100,000/QALY.