© 2014 Ankara University. All rights reserved.Materials and Methods: Serum YKL-40 levels in venous blood samples of 55 patients was studied. 49 (89.1%) male and 6 (10.9%) female newly diagnosed NSCLC patients whom received neither cancer specific or symptom relieving treatment are enrolled. TNM staging was determined based on the findings of computerized tomography (CT), positron emission tomography (PET), cranial magnetic resonance imaging (MRI), bronchoscopy and mediastinoscopy. The patients with NSCLC were divided into two groups; Group A: stage Ia, Ib, IIa, IIb, Group B: stage IIIa, IIIb and IV.Results: There was a statistical difference in YKL-40 serum levels between groups (Group A, Group B) when compared (p<0.05). A medium and statistical correlation was found (r= 0.48; p<0.01) between YKL-40 levels and age.Conclusion: YKL-40 levels in advanced stage NSCLC (stage III, IV) was found to be significantly high compared to early stage. It should be kept in mind that when YKL levels are high, higher stages of the disease should be suspected and future tests should be performed.Introduction: Despite varies treatment options, the survival rate of non-small cell lung cancer (NSCLC) is less than 1%. If biological behaviour of any cancer could be known, the information would potentially tailor the clinical work-up, assesment and treatment. The prognostic value of serum YKL-40 level has been investigated in different types of cancer and showed poor prognostic indication with more aggressive clinical course. We studied the role of serum YKL-40 levels in patients with NSCLC to determine the stage.