Tryptophan degradation metabolites are known to suppress T-cell function, which is a mechanism of resistance of tumor cells against immune surveillance. The aim of this study was to evaluate tryptophan degradation along with serum neopterin levels in benign and malignant breast disease. Serum tryptophan and kynurenine levels and neopterin concentrations of 30 patients with malignant and 27 patients with benign breast disease were determined by HPLC and ELISA, respectively. The slight increase in tryptophan degradation in a subgroup of cancer patients with higher grade tumors was not statistically significant, but the increased degradation was correlated with higher neopterin concentrations. Neopterin levels in patients with malignant breast disease were significantly higher than in the benign group (p<0.05). Tryptophan degradation positively correlates with the aggressiveness of the tumor because it changes with tumor grade rather than disease stage.