A 56-year-old man presented with a chief complaint of chronic cough due to bronchial asthma and pulmonary emphysema in 2001, without any abnormal findings on chest CT. His symptoms improved with high-dose inhaled corticosteroid. In February 2004, multiple nodules without bronchiectasis appeared in the chest CT. Pulmonary Mycobacterium avium infection was diagnosed by bronchial lavage and sputum culture. After multiple nodules appeared and disappeared repeatedly without medication, most nodules vanished after administration of antituberculous drugs. In Feburary 2007, a rapidly growing mass appeared in the right upper lobe, and a new nodule emerged in the left upper lobe the following month. On 18F-fluorodeoxyglucose positron emission tomography (18 FDG-PET), a substantial difference in 18FDG uptake was observed although both lesions were shown to be caused by Mycobacterium avium infection by needle biopsy. The lung specimen of the lesion with high 18FDG uptake demonstrated neutrophil infiltrates, suggesting acute inflammation. On the other hand, neutrophil infiltrates were not observed in the lesion with low uptake. We conclude that the degree of 18FDG uptake is not useful to decide when to initiate therapy and evaluate the efficacy of treatment.