We evaluated the significance of indoleamine 2, 3-dioxygenase (IDO) in recurrent breast cancer during chemotherapy. IDO activity can be measured by tryptophan (Trp)/kynurenine (Kyn) ratio. Trp and Kyn were measured by high performance liquid chromatography (HPLC). The correlations among age and Trp/Kyn ratio or immunosuppressive acidic protein (IAP) value in pre-chemotherapies or post-chemotherapies were studied. In under 35-year-old patients, there were no correlations between pre-chemotherapy and post-chemotherapy in IAP values and Trp/Kyn ratio. But in over 36-year-old patients, both Trp/Kyn ratio and IAP value in post-chemotherapy were higher than in pre-chemotherapy. These results suggest that the immunological damages for the patients during chemotherapy may depend on the age of patients.