Purpose: It is difficult to determine the activity of tuberculosis radiologically. Therefore there have been efforts to assess the activity using radiopharmaceuticals such as 67Ga, 99mTc-tetrofosmin, and 99mTc-MIBI. But there may be some discrepancy in defining the term 'activity' between clinicians and nuclear physicians. While negative conversion of sputum acid fast bacilli (AFB) is defined as 'disappearance of activity' by clinicians, a loss of uptake in previously positive lesion is accepted as 'disappearance of activity' by nuclear physicians. We designed a prospective study to see if the negative conversion of sputum AFB could directly match the disappearance of radioactivity of the lesion. Materials and Methods: Fifteen patients with bacteriologically confirmed active localized pulmonary tuberculosis were scanned 10 and 60 min after intravenous injection of 550 MBq 99mTc-MIBI. In 6 patients, who showed negative conversion of sputum AFB after 3-7 months of chemotherapy, 99mTc-MIBI scan was repeated. For the purpose of comparison, target/nontarget ratios of the lesions were determined. Results: 12/15 (80%) patients with active pulmonary tuberculosis showed increased uptake of 99mTc-MIBI in tuberculous lesion. After negative conversion of sputum AFB, 5/6 (83%) patients still showed increased uptake, although the intensity of uptake decreased. Conclusion: Uptake of radioactivity decreased but did not disappear after negative conversion of sputum AFB. 99mTc-MIBI scan may be useful to address the degree of inflammation of pulmonary tuberculous lesion, but the uptake did not directly match the activity defined by positivity of sputum AFB. We nuclear physicians might have used the term 'activity' somewhat differently from clinicians who treat patients with tuberculosis. (Korean J Nucl Med 2000;34:129-134)