The immune system of the patients with colorectal cancer is activated. Urinary neopterin is an index of this systemic immune activation, reflecting the circulating activity of interferon-gamma. We have measured urinary neopterin excretion in 34 patients with colorectal cancer. This excretion was significantly higher in our patients compared to reference group (325 +/- 267 vs 169 +/- 62 mumol/mol creatinine, neopterin concentrations were considerably higher in Dukes' stage D disease compared with Dukes' stage B and C disease/561 +/- 372 vs 188 +/- 47 and 250 +/- 142 mu/mol creatinine. Increased neopterin detected the presence of metastatic disease with 90% sensitivity and 87.5% specificity. Authors conclude that urinary neopterin measurement may be useful in the staging of colorectal cancer.