The lung adenocarcinoma is considered more aggressive than other types of non-small cell lung cancer. Asmetabolites of tryptophan degradation along the kynurenine pathway, including kynurenic acid, have been shownto induce immunosuppression and facilitate escape of tumor cells from immune surveillance, a hypothesis was setup that differences in biological behavior between types of lung cancer may be associated with altered activity ofthe kynurenine metabolic pathway. The aim of the study was to determine kynurenic acid levels in the serum ofpatients with bronchial adenocarcinoma for comparison with other types of non-small cell lung cancer. A totalof 227 patients with non-small cell lung cancer were enrolled in the study, including 71 with adenocarcinomaand 96 with squamous cell carcinoma. Serum kynurenic acid concentration was determined with use of highperformance liquid chromatography and fluorometry. The level of kynurenic acid in the serum of patients withadenocarcinoma was significantly higher than in those with squamous cell lung cancer (107.1 ± 62.8 pmol/ml;95%CI: 92.4 to 132.3 pmol/ml versus 82.1 ± 47.6 pmol/ml; 95%CI: 78.5 to 91.2 pmol/ml, respectively; p = 0.027).Differences between other histological types of lung cancer were insignificant. We conclude that increased activityof kynurenine metabolic pathway manifested by elevated serum kynurenic acid level may be one of the factorsassociated with clinically distinct biological behavior of adenocarcinoma, in particular high invasiveness andrapid progression.