BACKGROUND: Etiologic diagnosis of pleural effusion is usually made by clinical characteristics, pleural fluid analysis and pleural biopsy. But, despite careful diagnostic study, the cause of pleural effusion cannot be found in about 20 percent of patients, especially in loculated pleural effusions. Tuberculous pleurisy is one of the most common cause of pleural effusion in Korea. But, pleural fluid culture for Mycobacterium tuberculosis are positive in only 20 to 30 percent of patients and typical pleural biopsy finding in less than 50 percent of patients with this disease. In recent studies, adenosine deaminse(ADA) and its isoenzymes were proposed to be a useful diagnostic tool for differential diagnosis of pleural effusion We investigated the pattern of ADA and its isoenzyme activities in various cause of pleural effusions to evaluate the diagnostic value of measuring ADA and its isoenzymes. METHOD: We measured total ADA and its isoenzyme activities in pleural fluid and serum from 54 patients with pleural effusion(25 tuberculous pleural effusion, 10 parapneumonic effusion, 14 malignant pleural effusion, 5 transudative pleural effusion), including 5 loculated tuberculous pleural effusions and 6 loculated parapneumonic effusions. Total ADA activity was measured by the spectrophotometric method and ADA2 isoenzyme activity was measured with same method using EHNA, potent inhibitor of ADA1 isoenzyme activity. RESULT: Total ADA activity of tuberculous pleural effusion was higher than malignant pleural effusion(p<0.01), but no significant difference was found between tuberculous pleural effusion and parapneumonic effusion (tuberculous pleural effusion:148.9+/-9.91U/L, parapneumonic effusion:129.0+/-119.41U/L, malignant pleural effusion 48.7+/-9.71U/L). Percentage of ADA2 activity to total ADA activity(ADA2%) of pleural effusion of tuberculous pleurisy was higher than parapneumonic effusion(p<0.05), but no significant difference was found between tuberculous pleural effusion and malignant pleural effusion(tuberculous pleural effusion: 57.2+/-10.7%, parapneumonic effusion: 35.9+/-17.8%, malignant pleural effusion: 60.7+/-4.1%). In loculated pleural effusion, ADA2% of tuberculous pleural effusion was higher than parapneumoriic effusion(tuberculous pleural effusion: 53.3+/-3.9%, parapneumonic effusion: 27.8+/-7.9%). CONCLUSION: Measurement of ADA isoenzyme activity is useful for differentiating tuberculous pleural effusion from parapneumonic effusion, especially in loculated pleural effusion.