The objective of the study was to evaluate the information content of determining the activity of adenosine deaminase and adenosine deaminase-2 in the diagnosis of tuberculous pleurisy in patients with HIV infection.Materials and methods. A total of 378 patients with pleural effusion were retrospectively examined. In 215 cases, tuberculous pleurisy was detected (TP); and 163 patients had non-tuberculous pleural effusion (non-TP). As much as 27 patients in the TP group were HIV co-infected (TP/HIV+), the remaining 188 patients were HIV — negative (TP/HIV–). In all the patients, the activity of total adenosine deaminase (ADA) and its isoenzymes (ADA-1 and ADA-2) in the pleural fluid was determined.Results and discussion. In the TP group, the activity of total ADA (95.5 [67.7; 115.4] versus 82.0 [59.6; 100.0] U/L, p=0.1), ADA-1 (14.2 [5.8; 20.5] versus 12.1 [6.1; 23.7] U/L, p=0.9) and ADA-2 (78,1 [38.1; 93.1] versus 62.4 [35.4; 82.2] U/L, p=0,1) did not depend on HIV status. The activity of these indicators was determined above the threshold level — total ADA in 96.3% and 95.2%, ADA-1 in 25.9% and 30.8% and ADA-2 in 92.6% and 83.3% of cases in the «TP/HIV+» and «TP/HIV–» groups, respectively. A negative correlation between ADA-1 activity and HIV viral load in the group of patients with tuberculous pleurisy and HIV infection (r=–0.45; p=0.008), as well as in the subgroup of TP/HIV+ patients who received (r=–0.9; p=0.008) and in those who didn’t receive ART (r=–0.47; p=0.04) was obtained. Our results show that a total ADA activity increase in the patients with tuberculous pleurisy, regardless of patients’ HIV status, occur due to ADA-2. Thus, the increase in activity of total ADA and ADA-2 in our study was caused by active tuberculosis, not by the presence or absence of HIV co-infection. Also, the ADA-2 activity in HIV-infected patients is likely consistent with ADA-2 important role in cellular immune responses.Conclusion. Our data indicate the participation of purine metabolism enzymes in the pathogenesis of HIV infection. At the same time, adenosine deaminase activity is not a specific biomarker of individual changes characteristic of HIV infection. The study results suggest that the total adenosine deaminase and adenosine deaminase-2 activity increase is a valuable and diagnostically significant marker of tuberculous pleurisy in HIV-infected patients. The value of adenosine deaminase and adenosine deaminase-2 activity remains high even in the patients having severe immunosuppression, which allows them to be actively used for rapid diagnostics and hence, early TB therapy initiation.