The aim of this study was to determine the importance of serum neopterin level in female patients with breast cancer of various clinical stages. The study consisted of 75 female patients with breast cancer who were diagnosed and treated at the Gazi University Department of Medical Oncology. The patients were classified into three representative groups and a control group: group A (n = 26), patients with newly diagnosed primary breast cancer and without metastasis; group B (n = 33), patients with metastatic breast cancer who had undergone treatment for their diseases and on whom metastasis was detected during their follow-up; group C (n = 16), off-therapy patient whose cancer had been in remission for at least 5 years; group D (n = 20) healthy controls. The median serum neopterin levels of the 75 patients with breast cancer 11.0 (range, 0-23.6) nmol/L were significantly higher than those of controls (8.3 (range, 1.2-12.0) nmol/L). In group B patients, neopterin levels (12.6 (range, 0-23.6) nmol/L) were statistically significantly higher than those of controls, primary breast cancer patients, and off-therapy patients (P < .05). In group B, patients with visceral metastases had higher neopterin levels than did those with bone or local metastases; however, that difference was not statistically significant. The median serum neopterin levels of the primary breast cancer patients in group A (8.8 (range, 0-20) nmol/l) were not statistically significantly different from those in controls and off-therapy patients. Serum neopterin levels were significantly elevated in patients with metastatic breast cancer. Neopterin seems to be an indicator of metastatic cancer rather than a marker for local cancer. In patients with metastatic breast cancer, determining the serum neopterin levels may be useful in estimating survival; however, additional long-term follow-up will be needed.