Lentinula edodes mycelia extract (L.E.M.) is extensively utilized as an herbal medicine. However, its safety and effectiveness have not yet been scientifically verified. In this study, we investigated its safety and its influence on quality of life (QOL) and the immune response in patients undergoing cancer chemotherapy. Seven patients were studied in total. The patients were undergoing postoperative adjuvant chemotherapy for breast cancer (n = 3) or gastrointestinal cancer (n = 2), or were receiving chemotherapy to prevent recurrence of gastrointestinal cancer (n = 2). The first course of treatment was chemotherapy alone and the second was chemotherapy plus concomitant administration of L.E.M. Adverse events and changes in the QOL score, lymphocyte subpopulations, lymphocyte activity and serum immune indices were evaluated during the study period. No adverse events attributable to L.E.M. were observed. Compared to the pre-chemotherapy state, no changes in QOL or immune parameters were noted after the first chemotherapy course. In contrast, following the second course of combined therapy, improvements were noted in QOL (p < 0.05), NK cell activity (p < 0.05) and immunosuppressive acidic protein (IAP) (p < 0.01) levels. Although a future large-scale investigation is necessary to confirm these results, these data suggest that the concomitant of L.E.M. with chemotherapy is safe and improves the QOL and immune function of patients undergoing chemotherapy.