OBJECTIVES: We investigated the effect of adjuvant chemotherapy regimens on utility scores assessed by the EQ-5D instrument in a randomized controlled trial for breast cancer patients after surgery. METHODS: In the National Surgical Adjuvant Study of Breast Cancer-02 (N-SAS BC 02), 1,060 patients were randomly assigned to the following 4 chemotherapy groups: a) 4 cycles of anthracycline (ADM 60mg/m2 or EPR 75mg/m2 + CPM 600mg/m2, q3 wks x 4) followed by paclitaxel (175 mg/m2, q3 wks x 4) (ACP), b) 4 cycles of anthracycline followed by docetaxel (75 mg/m2 q3, wks x 4) (ACD), c) 8 cycles of paclitaxel (175 mg/m2, q3 wks x 8) (PTX), and d) 8 cycles of docetaxel (75 mg/m2, q3 wks x 8). The first consecutively registered 300 women were the subjects of the present utility study. Utility scores were assessed using the EQ-5D instrument at baseline, 3rd cycle, 5th cycle, 7th cycle, 7 months, and 1 year. The obtained data were analyzed using a linear mixed model with baseline, time, group, and interaction between time and group as explanatory variables. RESULTS: Missing data was observed between 1.9 and 6.1% of cases depending on the time of measurement. The utility score was significantly lower in the DTX group than in the ACP and ACD groups. In the DTX group the mean utility score was lowest at 7 months, and it tended to remain low for a long time. In a comparison of the anthracycline and taxane groups, the anthracycline group had significantly higher utility scores. There were no significant differences depending on the type of taxane. The estimated mean utility scores were 0.81, 0.83, 0.79, and 0.76 (ACP, ACD, PTX, and DTX group). CONCLUSIONS: The results of this study will be beneficial not only for clinical decision-making but also for appropriate allocation of medical resources.