This study was done to investigate the course of serum adenosine deaminase (sADA) level during pulmonary tuberculosis (TB) and its relationship with clinical, radiological, and laboratory parameters in TB patients. sADA levels were measured at the beginning and after the first, second, and sixth months in 38 smear-positive TB patients. Chest X-rays were taken. Additionally, peripheral blood leukocyte and lymphocyte counts, erythrocyte sedimentation rate (ESR), and C reactive protein (CRP) levels were measured. Fifteen healthy persons were enrolled in the study as controls. The level of sADA at the beginning was 33.2 +/- 13.9 U/1 in the patients and 19.0 +/- 7.1 U/1 in controls. The specificity and sensitivity were found to be 100% and 42%, respectively. The level of sADA showed elevation (45.1 +/- 10.6 U/1) after one month and gradually decreased in the second (34.6 +/- 10.1 U/1) and sixth months (24.6 +/- 4.7 U/1). ANOVA (post hoc Bonferroni) showed a significant difference in sADA levels between the beginning and the first month (P=0.005), the first and second months (p=0.016), and between the first month and the end of treatment (P < 0.001). There was also a significant difference between sADA level at the end of treatment and the control value (p=0.01). During these times there were significant differences in the radiological course, peripheral blood lymphocyte count, ESR, and CRP levels. sADA levels in TB patients showed a slight elevation in the first month, but it decreased during treatment in parallel with the effectiveness. This may have an association with lymphocytic activation.