The aim of the present study was to evaluate the relationship between the levels of neopterin among patients with benign and malignant breast disease and the relation with the stage of the malignant process. In this study, neopterin concentrations and enzyme activities of superoxide dismutase (SOD) and catalase (CAT) were determined in malign (n=30) and benign breast tumor patients (n=30) by high performance liquid chromatographic and spectrophotometric methods, respectively. Results were compared with a healthy control group (n=20). The correlations between neopterin, CAT and SOD were also evaluated in controls and patients. Urinary neopterin level of the control group was (mean value +/- S. D.) 128.6 +/- 64.6 mu mol/mol creatinine. Neopterin concentrations in patients with breast malignancy were 153.6 +/- 71.2 mu mol/mol creatinine and 107.8 +/- 32.1 mu mol/mol creatinine in benign disorders patients. The mean neopterin level in the benign group was found to be statistically different from the malign tumor group (p = 0.039). SOD and CAT activities in controls were found as 3.57 +/- 0.84 U/mg protein and 2.19 +/- 0.20 U/mg protein, respectively. In patients with malignancy, the SOD activity was 3.84 +/- 0.73 U/mg protein while CAT activity was 1.03 +/- 0.13 U/mg protein. Patients with benign breast disorders, SOD activity was 4.09 +/- 1.00 U/mg protein and CAT activity was 1.02 +/- 0.18 U/mg protein. Whereas SOD activity did not differ between the groups of patients and controls, the mean catalase level in the control group was higher than in the benign and malign tumor groups (both p < 0.001). Urinary neopterin concentration seems to be an important and useful biomarker in diagnosis of breast tumors in clinical practice.