This study assesses cost of illness of metastatic breast cancer (MBC) patients over a specific period of time as observed in Medicare program expenditure data. Incident cases of MBC from 1997 to 1999 were included. Direct care cost data were obtained from Medicare claims from a 5% national sample of program beneficiaries. Costs to the health care system were determined from the date of first diagnosis of MBC to the end of follow-up. A matched case-control study provided comparison of Medicare payments in cancer and non-cancer patients. A total of 397 MBC patients were identified and followed for an average of 16.2 months. The mean total cost was US$ 35,164 per MBC patient and US$ 4176 per person for the control group. Over the follow-up period, the MBC patients averaged 1.7 inpatient admissions per patient and 14.4 inpatient days per admission. The control group averaged 0.3 inpatient admissions per patient and 1.6 inpatient days per admission. Home health aide for MBC averaged one visit every 2 weeks. The direct costs for the older MBC patients were less than the direct costs of the younger MBC group. Multivariate regression analysis showed that as the age increased, the Medicare costs decreased within the MBC cohorts. Less combination treatments were provided for older MBC patients. Cost of illness was inversely proportional to age.