Extract of Lentinula edodes mycelia (LEM) is currently utilized as an oral biological response modifier (BRM) medicine for cancer patients. However, its effectiveness for breast cancer patients with postoperative adjuvant hormone therapy has not yet been scientifically verified. In this study, we investigated the influence of LEM on the quality of life (QOL) and immune response in breast cancer patients undergoing postoperative adjuvant hormone therapy. Twenty patients were studied in total. They received only hormone therapy in the first 4 weeks followed by hormone therapy and LEM during the next 8 weeks. Laboratory tests, QOL score and peripheral blood cytokine production levels were evaluated during the study period. No changes in QOL or cytokines were noted after the first 4 weeks. In contrast, during the following combined therapy period, improvements were noted in QOL and cytokine levels. Although a future large-scale investigation is necessary to confirm these results, these data suggest that the concomitant use of LEM with postoperative adjuvant hormone therapy improves the QOL and immune function of patients.