We evaluated the significance of indoleamine 2,3-dioxygenase (IDO) in recurrent breast cancer patients 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 Trp/Kyn ratio and immunosuppressive acidic protein (IAP) value in pre-chemotherapies or post-chemotherapies were studied. There were no correlations between pre-chemotherapy and post-chemotherapy in IAP values and Trp/Kyn ratio with weekly paclitaxel therapy. And there were no correlations between pre-chemotherapy and post-chemotherapy in IAP values, but Trp/Kyn ratio in post-chemotherapy was higher than re-chemotherapy with tri-weekly docetaxel therapy. These results suggest that the weekly paclitaxel therapy may be less invasive for recurrent breast cancer patients than the tri-weekly docetaxel therapy.