OBJECTIVES Trastuzumab (Herceptin) was approved in 1998 for treating patients with metastatic breast cancer. However, there is little information on the cost of care in these patients. The following abstract quantifies the costs for these patients. METHODS We used SEER-Medicare data to identify inpatient, outpatient, and total costs, in women diagnosed with metastatic breast cancer- Stage IV (S-IV) or Stage 0-III with distant recurrence (S-DR) who received trastuzumab. An index date was defined as either the date of diagnosis (S-IV) or of first distant recurrence (S-DR). Included patients were diagnosed in 2000-2002, and had their first claim for trastuzumab between their index date and December 31, 2005, the end of the observation period. Patients were divided into those who received trastuzumab as part of their first treatment following their index date (Group A), and those who began trastuzumab after at least one course of chemotherapy (Group B). Monthly inpatient, outpatient, and total Medicare costs were calculated and adjusted to 2008 dollars using the Medicare component of the CPI. RESULTS A total of 281 patients met the inclusion criteria, of whom 100 (36%) were diagnosed as stage IV at index (N=181 for stage 0-III with distant recurrence) and 191 patients received trastuzumab as part of initial treatment. For the group in general, the average total monthly cost was $6104- 29% due to inpatient costs ($1744). In bivariate analysis, the average cost for S-IV patients ($6762) was significantly higher (p=0.03) than for S-DR patients ($5740), largely due to inpatient costs ($2253 versus $1463). The average monthly cost of care was significantly higher (p=0.004) in Group A ($6538) than Group B ($5168). These differences persisted in multivariate analyses. CONCLUSIONS Medicare costs are greater among women initially diagnosed with Stage IV breast cancer, compared to those diagnosed with Stage 0-III disease who have a distant recurrence.