BACKGROUND: The purpose of this study was to evaluate the usefulness of direct PCR for a rapid detection of Mycobacterium tuberculosis (MTB), the differentiation of MTB from nontuberculous mycobacteria (NTM), and the identification of NTM species in acid-fast bacilli (AFB) smear-positive specimens. METHODS: A total of 255 AFB smear-positive respiratory specimens were studied. For the differentiation of MTB from NTM, the bands of the 360 bp, which exists both in MTB and NTM, and the 190 bp, which exists only in MTB, were amplified using the onetube nested PCR targeting regions of the rpoB gene. The two-tube nested PCR targeting 16S-23S rRNA spacer gene was done with 34 specimens that were negative by one-tube nested PCR. The specimens that were tested positive for NTMs were subsequently subjected to PCR-restriction fragment length polymorphism (PCR-RFLP) analysis based on the rpoB gene for mycobacterial species identification. RESULTS: Detection rate of MTB and NTM was 87% after the one-tube nested PCR. The detection range of MTB and NTM increased up to 93% after the two-tube nested PCR. The results of PCR-RFLP analysis identified those NTMs as M. avium and M. intracellulare. CONCLUSION: This result seems to suggest strongly that the PCR testing especially aiming for differentiation of MTB from NTM, and identificatin of mycobacterial species using AFB smear-positive specimens would be highly importnat in clinical settings for effective treatment of patients.