This study investigated the influence of Lentinula edodes mycelia extract (LEM), an oral immunomodulator, on immune function and adverse events from chemotherapy. Subjects comprised 1 gastric and 7 colorectal cancer patients. The first course of treatment was chemotherapy alone and the second was chemotherapy plus concomitant administration of LEM. Adverse events and interferon (IFN)-γ production by CD4+ T, CD8+ T and CD56+ NK/NKT cells were evaluated at the end of each course. Grade 1 or 2 adverse events were observed at the end of the first course for 6 of 8 patients. In comparison, no patients displayed any adverse events at the end of the second course. Tendencies toward improved IFN-γ production by CD4+ T, CD8+ T and CD56+ NK/NKT cells was also seen. These results suggest that concomitant use of LEM with chemotherapy can decrease the incidence of adverse effects from cancer chemotherapy among patients with advanced cancer.