To investigate the factors influencing the quality of life (QOL) of Japanese patients with advanced or recurrent breast cancer, a newly developed QOL questionnaire, "The QOL Questionnaire for Cancer Patients Treated with Anticancer Drugs" (QOL-ACD), was answered by 23 patients, and a multiple regression analysis was performed. The demographic and medical factors relating to the overall QOL score and to the four categories of the QOL-ACD, namely (1) activity, (2) physical condition, (3) psychological condition, and (4) social relationships, were analyzed. The results indicated that skin metastasis, a heavier body weight, and bone metastasis had a strong negative influence on the overall QOL scroe, whereas endocrine therapy, the existence of a primary lesion, and more extensive first surgery had a strong positive influence on it. With regard to the analysis of the four categories, endocrine therapy was found to be positively related to all four categories. The multiple correlation coefficient (R) between the estimated overall QOL score and the observed overall QOL score was about 0.77. The results of this analysis showed that endocrine therapy can improve the QOL of patients with advanced or recurrent breast cancer, and that the QOL-ACD questionnaire could prove extremely useful for predicting the QOL of individual patients and for aiding clinicians in deciding on the most appropriate type of therapy for each patient.