SUMMARY Adenosine Deaminase Activity (ADA) levels were determined in Bronchoalveolar Lavage Fluid (ΒALF) of 102 patients, aged 24-78 years (63 men,39 women) whο underwent bronchoscopy in order to establish diagnosis for various pulmonary diseases. Fifty patients with active pulmonary Tuberculosis were further divided into three subgroups according to the radiologic extent of lesions: A for the extended disease, B for the less extended disease and C for the limited disease. Fifty two patients with other pulmonary diseases were used as control group for the comparisons with Tuberculosis group: Fourteen with Bronchogenic Carcinoma, 16 with Interstitial Pulmonary Disease (UIP), 10 with Sarcoidosis and 12 with non specific Infections of the lower respiratory tract. ADA levels were measured in ΒALF as well as in blood serum. Cellular subpopulations and percentages as well as CD4/CD8+ ratio were also determined in BALF in 20 patients of the Tuberculosis group. The levels of ADA in BALF of patients with Tuberculosis were significantly higher (p<0,001) compared to the other groups of patients with the exception of patients with Sarcoidosis (p=0,09). ADA values in ΒALF were significantly different in the three subgroups A,B and C (p<0,001), well correlated to the extent of the chest x-ray lesions. Sensitivity and Specificity of ADA in ΒALF for the diagnosis of Tuberculosis were 80% and 78,6% respectively for cut off value of 1,85u/lt, excluding Sarcoidosis patients. Sensitivity and Specificity of ADA in ΒALF were higher in group A compared to B or C. An important increase of Lymphocyte and Neutrophile percentages (20,5 ± 11,6) and (4,2% ± 5,6) respectively as well as an important increase of CD4+/CD8+ ratio (4,9 ± 2,3) were observed in BALF of patients with Tuberculosis. ADA was strongly correlated with CD4+/CD8+ ratio values showing a tendency of almost parallel increase. Our results indicated that ADA determination in ΒALF constitutes a usefull marker in the diagnosis of Tuberculosis especially when radiologic lesions are extended. Also cellular measurements in BALF of patients with Tuberculosis indicated that Adenosine Deaminase plays the role of a marker of the immunological response of the lung infected by Mycobacterium Tuberculosis. Pneumon 1999, 12 (2): 104-116