Anti-TNF-α agents are increasingly used in rheumatoid arthritis (RA) treatment and that is known to increase the risk of tuberculosis (TB) reactivation. Adenosine deaminase (ADA) levels are shown to increase to high levels in TB patients. Our aim is to investigate the serum ADA levels in RA patients being treated with anti-TNF-α and to compare the results with the patients on DMARD therapy. The study groups comprised of 56 RA patients (45 female, mean age 49) who were treated either with two or three DMARDs, 32 RA patients with anti-TNF-α treatment (26 female, mean age 46) and 20 healthy controls (10 female, mean age 48). All patients fulfilled the 1987 ACR criteria for RA. DAS28 score was calculated for all subjects. When compared to healthy controls, ADA levels were measured statistically higher both in patient groups (P = 0.046, 0.002). ADA levels in anti-TNF-α group were similar to conventional therapy (11.3 ± 2.7, 10.9 ± 4.01; P = 0.76). PPD was positive in 17 RA patients in the anti-TNF-α treatment group (%53). The ADA levels were found to be similar in the anti-TNF-α group when compared according to the PPD positivity (positive, 12.4 ± 3.7; negative, 10.5 ± 2.1; P = 0.02). No correlation was found between the ADA levels and age, disease duration, ESR, CRP, DAS 28 and HAQ score. In this study, we observed that RA patients at remission taking DMARD or anti-TNF-α therapy have similar levels of serum ADA. Although serum ADA levels during TB infection increase much higher, in our study, ADA levels of all RA patients were lower than 15 IU/L. Elevated ADA levels may be a clue for diagnosis of TB in patients who were on anti-TNF-α therapy. © 2008 Springer-Verlag.