OBJECTIVES To characterize health care resource use and costs in US women with incident metastatic breast cancer receiving chemotherapy as their principal treatment modality. METHODS Using a retrospective cohort design and a large private health insurance claims database (2000-2005), we identified all women initiating chemotherapy for incident metastatic breast cancer with no evidence of concomitant or subsequent receipt of hormonal therapy or trastuzumab. Health care resource use (inpatient, outpatient, medications) and costs were tallied on a cumulative basis from date of chemotherapy initiation (ie, index date) to date of disenrollment from the health plan (in most instances, presumably due to death) or the end of the study period, whichever occurred first. Study measures were summarized using Kaplan-Meier Sample Average (KMSA) method; 95% CIs were generated using nonparametric bootstrapping. Reimbursed amounts were used as a proxy for costs. RESULTS The study population consisted of 820 women; mean (SD) age was 58.4 (12.0) years. Over a mean follow-up of 692 days (range: 3 to 2,182), study subjects averaged 1.1 hospital admissions, 6.8 inpatient days, and 62.4 physician office and hospital outpatient visits. Mean (95% CI) cumulative total health care costs were $91,400 ($83,804, $99,050) per patient over this period. Outpatient chemotherapy constituted 24% of total health care costs; comparable percentages for inpatient care, outpatient services, and all other outpatient pharmacotherapy were 19%, 32%, and 24%, respectively. CONCLUSIONS Health care costs are high in US women with incident metastatic breast cancer receiving chemotherapy as their principal treatment modality. This study provides important additional information on the cost of treatment of this disease.