Background: Tuberculous pleurisy is the leading cause of pleural effusion in Korea. And differential diagnosis of tuberculous pleurisy with other cause is clinically very important. Traditional diagnostic methods such as routine analysis of pleural fluid, staining for acid-fast bacilli or pleural biopsy have major inherent limitaion. This study was designed to evaluate the significance of pleural fluid polymerase chain reaction(PCR) and adenosine deaminase (ADA) activity in early diagnosis of tuberculous pleurisy. Material and Method: Between March 1996 and July 1997, 198 patients with pleural effusion reviewed retrospectively. The study group included 112 cases with tuberculous effusion and 86 cases with non-tuberculous effusions, whose diagnoses were confirmed by pleural biopsy, microbiological methods, or cytology. We compared the results of PCR and pleural fluid levels of ADA between tuberculous and non-tuberculous effusions. Result: Mean age was 47.54$pm$19.52 years(range 2 to 85 years). The positive rate of PCR was significantly higher in tuberculous group than non-tuberculous group(p<0.05). The sensitivty, specificity, positive predictive value(PPV), and negative predictive value(NPV) for PCR were 31.7, 90.9, 83.0, and 48.8%, respectively. Mean ADA activity was significantly higher in tuberculous group than non-tuberculous group(83.2 U/L vs 49.8 U/L)(p<0.05). With diagnostic thresholds of 40 U/L, the sensitivity, specificity, PPV, and NPV of ADA for tuberculosis were 75.9, 70.9, 77.3, and 69.3% respectively. At a level of 70 U/L, the sensitivity, specificity, PPV, and NPV of ADA for tuberculosis were 70.1, 75.9, 82.9, and 60.3% respectively. Conclusion: PCR is very highly specific, but less sensitive methods in diagnosis of tuberculous pleurisy. But ADA level of pleural fluid has acceptable sensitivity and specificity in diagnosis of tuberculous pleurisy. ADA activity is more useful test in the evaluation of pleural effusions.