Endocrine therapy is effective for elderly patients who are diagnosed with estrogen receptor-positive breast cancer and cannot receive surgical treatment or chemotherapy because of advanced age. The duration of response in patients receiving letrozole is approximately 10.3 months. There are few cases with more than 3 years of response. We evaluated the significance of indoleamine 2,3-dioxygenase(IDO)during letrozole therapy in long-term response. We measured IDO activity based on the tryptophan(Trp)/kynurenine(Kyn)ratio. Trp and Kyn levels were measured using high-performance liquid chromatography(HPLC). The Trp/Kyn ratio decreased along with tumor reduction after letrozole therapy. These results suggest that measuring the Trp/Kyn ratio may be useful for evaluating immunological status during endocrine therapy in elderly patients with locally advanced breast cancer.