Protein-bound polysaccharide K (PSK) increased the 5-year disease-free survival rate and reduced the risk of recurrence in a randomised, controlled study for stage II and III colorectal cancer. In order to elucidate the disease-free survival benefits with PSK and what immunological markers could indicate a PSK responder, serial changes in immunological parameters were monitored in the study. PSK decreased the mean serum immunosuppressive acidic protein (IAP) level, and increased the mean population of natural killer (NK) cells compared with the controls. The 5-year disease-free and overall survival rate for patients with serum IAP values <or=500 microg ml(-1), which represents the normal value, were 75.5% (95% CI: 66.8-84.2%; p=0.016) and 85.1% (95% CI: 77.9-92.3%; p=0.032), respectively, in the PSK group compared with 57.5% (95% CI: 43.3-71.6%) and 70.2% (95% CI: 57.1-83.3%) in the control group. In patients with NK cell population >or=8% at 3 months after surgery, PSK conferred a significantly better (p=0.038) 5-year disease-free survival (86.7%; 95% CI: 74.5-98.8%) compared to the control group (60.0%; 95% CI: 29.6-90.4%). In the proportional hazards model, the presence of regional metastases (relative risk, 3.595; 95% CI: 1.518 to 8.518; p=0.004) and omission of PSK treatment (relative risk, 3.099; 95% CI: 1.202 to 7.990; p=0.019) were significant indicators of recurrence. PSK acts as an immunomodulatory activity and biochemical modulator in stage II or III colorectal cancer. Pre-operative serum IAP values <or=500 microg ml(-1) and an NK cell population >or=8% at 3 months after surgery are possible PSK response predictors.