BACKGROUND: As living and dead Mycobacteria could be amplified by polymerase chain reaction(PCR), it was considered that PCR was inappropriate for the monitoring of pulmonary tuberculosis after treatment. But we found negative conversion of PCR after successful treatment. We would like to know about the negative conversion rate of PCR and its conversion time after antituberculous treatment. METHODS: We collected 113 sputums from the 16 patients of pulmonary tuberculsosis visiting Catholic University Hospital of Taegu Hyosung. We consecutively tested AFB smear, AFB culture and PCR by 2 to 4 weeks after antituberculous therapy. The patients were classified according 13 the chest X ray findings. RESULTS: We detectsd negative conversion of PCR from all 16 patients of the pulmonary tuberculosis within 30 weeks after treatment. The average negative conversion time was 16 +/- 8 weeks. The conversion time according to the chest X-ray findings were as follows : For the 8 cases of minimum were 9 +/- 5 weeks,4 cases of modreate advanced were 20 +/- 8 weeks, and 4 cases of far advanced were 23 +/- 2 weeks. The product of PCR was gradually decreased according to the duration of treatment. CONCLUSIONS: From the results of our study, we could utilize M. tubercuosis PCR for the prediction of therapy response and monitoring of the patient with pulmonary tuberculosis after treatment.