Maitake D-Fraction, extracted from maitake mushroom, has been reported to exert its antitumor effect in tumor-bearing mice by enhancing the immune system through activation of macrophages, T cells, and natural killer (NK) cells. In a previous study, the combination of immunotherapy with the maitake D-Fraction and chemotherapy suggested that the D-Fraction may have the potential to decrease the size of lung, liver, and breast tumors in cancer patients. In the present study, we administered maitake D-Fraction to cancer patients without anticancer drugs, and at the same time NK cell activity was monitored to investigate whether the activity is closely related with disease progression. The numbers of CD4(+) and CD8(+) cells in the peripheral blood were measured in 10 patients, and NK cell activity was assessed using K-562 cells as target cells. Serum soluble interleukin-2 receptor (sIL-2R) levels in three patients and the expression of tumor markers in four patients were determined by enzyme-linked immunosorbent assay. The slight changes observed in the CD4(+) and CD8(+) cell numbers were independent of disease severity or stage as well as serum sIL-2R levels. In contrast, maitake D-Fraction hindered metastatic progress, lessened the expression of tumor markers, and increased NK cell activity in all patients examined. Thus maitake D-Fraction appears to repress cancer progression and primarily exerts its effect through stimulation of NK activity. In addition, we conclude that measurement of NK cell activity may be a useful clinical parameter in monitoring disease progression during and following immunotherapy with maitake D-Fraction.