BACKGROUND Lung cancer is both the most common seen malignity and cause of cancer-related deaths worldwide. Adenosine deaminase (ADA) is a hydrolytic enzyme that catalyses the conversion of adenosine to inosine in the purine metabolism pathway. Studies examining ADA levels in bronchoalveolar lavage (BAL) fluids of patients with lung malignancy are very limited in the literature. Our aim examine the clinical significance of ADA levels in BAL fluids of patients with lung malignancy. METHODS A total of 89 patients undergoing fiberoptic bronchoscopy (FOB) with different indications from December 2017 to December 2018 were included in this study. The patients were divided into two groups as malignancy and non-malignancy groups. Demographic, laboratory data and ADA levels in bronchoalveolar lavage (BAL) were compared between the two groups. In addition, ADA levels in BAL were compared among the histopathological subtypes of patients in the malignant group. RESULTS The mean age of the patients was 58.2 ± 14.5 years with 86% of male gender. ADA enzyme levels were statistically higher in the malignant patient group compared with the non-malignant group (37.2 [17.6-71] vs 17.1 [9-35.3], P < .001). When the patients in the malignant group were compared in terms of ADA levels according to their histopathological types, a statistically significant difference was obtained in small cell carcinoma patients (49 [12.5-75.3], P = .005). CONCLUSIONS ADA levels in BAL may be a diagnostic biomarker in lung malignancies. In patients where a biopsy cannot be taken or histopathological typing cannot be performed because of tissue insufficiency, ADA levels in BAL can be an auxiliary parameter in making malignancy / histopathological diagnosis accompanied by radiological and clinical findings.