We evaluated the clinical significance of indoleamine 2,3-dioxygenase(IDO)during letrozol therapy for an elderly patient with locally advanced breast cancer. 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 of the patient before endocrine therapy were lower than those after endocrine therapy. IDO activity decreased after endocrine therapy and correlat- ed with the number of metastatic lymph node lesions during letrozol therapy. These results suggest that measuring the Trp/ Kyn ratio may be useful for evaluating immunological metastatic status during endocrine therapy in elderly patients with locally advanced breast cancer.