Adenosine deaminase (ADA) is a widely used marker in the differential diagnosis of tuberculous effusion and there is evidence that its production is linked to Τ cells and monocytes. Both cell types are present in tuberculous effusions but are also present in malignant effusions where ADA activity is low. Furthermore, data on the correlation between ADA and Τ cells in tuberculous effusions are conflicting and based on small numbers of patients. We undertook this study to examine cell subsets in tuberculous effusion and their correlation to ADA. Pleural fluid from 73 patients was examined (thirty-seven patients with tuberculous and thirty-six patients with malignant effusion). The ΑΡΑΑΡ immunocytochemical method was used to examine Τ cells (CD3, CD4, CDB, CD25) and macrophages (CD68) while ADA activity was measured by the Guisti colometric method. Our results showed that CD3+, CD4+ and CD25+ cells and ADA were significantly higher in tuberculous effusion (p<0.001 for all measurements) while CD68+ were significantly lower (p<0.001). Νο differences were noted in CD68+ counts. Νο correlation was found betwween ADA and CD3 (r=0.18), CD4 (r=0.05), CD8 (r=0.09), CD25 (r=0.06), CD68 (r=0.04). Tuberculous effusion is characterized by high ADA activity and increased numbers of CD4+ Τ cells, but no correlation exists between ADA activity and cell numbers. Pneumon 2000, 13 (2): 154-160