BACKGROUND: Since polymerase chain reaction(PCR) was devised by Saiki in 1985, it has been used extensively in various fields of molecular biology. Clinically, PCR is especially useful in situation when microbiological or serological diagnosis is limited by scanty amount of causative agents. Thus, PCR can provide rapid and sensitive way of detecting M. tuberculosis in tuberculosis pleurisy which is diagnosed in only about 60% of cases by conventional method. METHOD: To evaluate the diagnostic usefulness of PCR in tuberculosis pleurisy, The results of PCR was compared with those of conventional method, including pleural biopsy. The pleural effusion fluid was collected from 7 proven patients, 7 clinically suspected patients and control group(7 patients with malignant effusion). We extracted DNA from pleural fluid by modified method of Eisennach method(1991). The amplification target for PCR was 123 base pair DNA, a part of IS6110. RESULT: 1) Sensitivity of PCR: We detected upto 50fg DNA. 2) In patients with pleural effusion of proven tuberculosis, the positive rate of PCR was 85.7% (6/7). In patients with pleural effusion of clinically suspected tuberculosis, the positive rate was 71.5% (5/7). In control group, positive rate was 0% (0/7). CONCLUSION: We concluded that PCR methd could be a very rapid, sensitive and specific one for diagnosis of M tuberculosis in pleural effusion. Further studies should be followed for the development of easier method.