Background: Urinary neopterin is an index of systemic immune activation, reflecting the activity of interferon-gamma. An increase in urinary neopterin excretion has been described in a variety of conditions, including malignant diseases, autoimmune diseases, or viral infections. Material and Methods: We measured urinary neopterin in 34 patients with colorectal cancer. In all patients the Dukes stage was determined. Neopterin was measured by high-performance liquid chromatography. Simultaneously, creatinine was determined by the Jaffé reaction and concentration of neopterin was expressed as neo-pterin/creatinine ratio (μmol/mol creatinine). The statistical analysis was performed by the Mann-Whitney U-test. Results: Neopterin excretion was significantly higher in the patients than in the reference group (325 ± 267 vs. 166 ± 67 μmol/mol creatinine, p < 0.005). Neopterin concentrations were considerably higher in stage Dukes D disease than in stages Dukes B and C disease (561 ± 372 vs. 188 ± 47 and 250 ± 142 μmol/mol creatinine, p < 0.001 and p < 0.002, respectively). An increased neopterin level detected the presence of metastatic disease with a sensitivity of 90 % and a specificity of 87.5 %. Conclusion: We conclude that increased urinary neopterin in colorectal cancer is mainly due to elevated levels in patients with metastatic disease.