We evaluated the clinical significance of indoleamine 2,3-dioxygenase(IDO)for breast cancer patients between different clinical stages and patient ages. IDO activity was measured by the tryptophan(Trp)/kynurenine(Kyn)ratio. Trp and Kyn levels were measured using high performance liquid chromatography(HPLC). Serum Trp/Kyn levels in Stage IV breast cancer patients were lower than in patients at other stages. Serum Trp/Kyn levels of breast cancer patients over the age of 70 years were lower than in patients under the age of 69 years. Within the same clinical stage, serum Trp/Kyn levels of breast cancer patients over the age of 70 years were lower than in patients under the age of 69 years. These results suggest that the immunological profile of breast cancer patients over the age of 70 years may be immunosuppressive compared to breast cancer patients under the age of 69 years.