FOLFOX4 has been proven to be effective for metastatic colorectal cancer and is now used as a postoperative adjuvant therapy. However, adverse effects such as cold-sensitive paresthesia and bone marrow suppression are common, and this may necessitate a change of chemotherapy regimen even though FOLFOX is effective. PSK, a polysaccharide derived from mushrooms, has been developed in Japan as an immune-enhancing agent, and is widely used in patients with gastric, colorectal and pulmonary cancer. PSK has also been reported to decrease some adverse effects of chemotherapy. FOLFOX4 combined with PSK was administered to patients with metastatic colorectal cancer and the results were evaluated. Eight cycles of FOLFOX4 and PSK (3.0 g/day, po) were given to 25 patients with metastatic (19 hepatic, 3 pulmonary and 3 peritoneal) colorectal cancer. There was no CR (0%), while PR, SD and PD were 48, 36 and 16%, respectively. The response rate was 48%, and the disease control rate was 84%. There were significantly lower frequencies of adverse effects in comparison with published data. Grades 1 and 3 neutropenia occurred in 48 and 24%, respectively, of the patients; grades 1 and 3 nausea in 48 and 4%; and grades 1, 2 and 3 sensory neurotoxicity in 52, 4 and 0%. No patient dropped out due to adverse effect in this study. PSK plus FOLFOX4 seemed to be as effective as FOLFOX4 monotherapy as has been published, and significantly less toxic. These results suggest that this combination therapy may be more effective than FOLFOX4 monotherapy when given over a longer period, with a lower incidence of adverse effects.