Context • Combined treatment with an extract of Lentinula edodes mycelia (LEM) and chemotherapy has been reported to improve quality of life (QOL) and immunological function in cancer patients. However, those effects have not been elucidated for patients receiving cancer immunotherapy. Objective • The present study intended to investigate the effects of oral LEM on QOL and immunological function in cancer patients receiving immunotherapy. Design • The research team designed an open-label, single-armed pilot study. Setting • The study took place at Bio-Thera Clinic, a facility associated with Tokyo Women's Medical University in Tokyo, Japan. Participants • The participants were 10 cancer patients undergoing cancer immunotherapy at Bio-Thera Clinic. Intervention • The participants received either dendritic cell (DC)-based cancer vaccine therapy or CD3-activated T-lymphocyte (CAT) therapy as immunotherapy. They received the immunotherapy only for the first 4 wk of the study, and then oral LEM (1800 mg/d) was added for the next 4 wk. Outcome Measures • Preintervention and at 4 and 8 wk after the start of the study, participants completed a QOL survey, and immunological parameters were measured. Results • Participants' QOL symptom scores increased (ie, worsened) by 5.1 ± 1.7 during the first 4 wk of treatment when they were receiving immunotherapy only, but it decreased (ie, improved) by -2.5 ± 1.6 during the next 4 wk when the immunotherapy was combined with the LEM, P < .05. The measurement of the immunological parameters during the 4 wk of immunotherapy combined with LEM showed that the amount of interferon-γ (IFN-γ) produced in the peripheral blood tended to increase as compared with that during the first 4 wk of immunotherapy only. The rise in IFN-γ was correlated with changes in several regulatory T cells (Tregs) (ie, forkhead box P3 [FOXP3]+/cluster of differentiation 4 [CD4]+ and transforming growth factor beta [TGF-β]). Conclusions • The findings suggest that a combined treatment of LEM and immunotherapy might improve QOL and immunological function in cancer patients.