Background: The long-term natural course of Mycobacterium avium complex (MAC) infection with nodular bronchiectasis is not well described and its therapeutic indications are not well established. This study was undertaken to identify the risk factors for deterioration of nodular bronchiectatic MAC lung disease during an over 5-year follow-up. Methods: Clinical and laboratory data of 67 patients with nodular bronchiectatic MAC lung disease were collected. Chest computerized tomographic images were used to count the number of involved lung segments at diagnosis and measure the subcutaneous fat thickness anteriorly to the xiphoid process during the follow-up (median [interquartile range] duration=80 [70-92] months). Results: Thirty-four patients (50.7%; 23 women; mean [standard deviation] age=59.5 [10.0] years) showed deterioration in this period. They had significantly lower body mass index (BMI; P=0.004) and % predicted forced vital capacity (P=0.032), higher numbers of involved lung segments (P＜0.001) and MAC-positive sputum cultures (P=0.028), and thinner chest subcutaneous fat during the follow-up (P＜0.009) than the patients without deterioration. In particular, the patients with both BMI ＜21.0 kg/m2 and involved lung segments ＞4 had a 240-fold increased risk of deterioration (95% confidence interval=13.7 to 4202.1; P＜0.001). Conclusions: We found that patients with poor nutritional status and wide lung involvement tend to show deterioration of nodular bronchiectatic MAC lung disease. Anti-MAC treatment may be considered for this high-risk group.is not well described and its therapeutic indications are not well established. This study was undertaken to identify the risk factors for deterioration of nodular bronchiectatic MAC lung disease during an over 5-year follow-up. Methods: Clinical and laboratory data of 67 patients with nodular bronchiectatic MAC lung disease were collected. Chest computerized tomographic images were used to count the number of involved lung segments at diagnosis and measure the subcutaneous fat thickness anteriorly to the xiphoid process during the follow-up (median [interquartile range] duration=80 [70-92] months). Results: Thirty-four patients (50.7%; 23 women; mean [standard deviation] age=59.5 [10.0] years) showed deterioration in this period. They had significantly lower body mass index (BMI; P=0.004) and % predicted forced vital capacity (P=0.032), higher numbers of involved lung segments (P＜0.001) and MAC-positive sputum cultures (P=0.028), and thinner chest subcutaneous fat during the follow-up (P＜0.009) than the patients without deterioration. In particular, the patients with both BMI ＜21.0 kg/m2 and involved lung segments ＞4 had a 240-fold increased risk of deterioration (95% confidence interval=13.7 to 4202.1; P＜0.001). Conclusions: We found that patients with poor nutritional status and wide lung involvement tend to show deterioration of nodular bronchiectatic MAC lung disease. Anti-MAC treatment may be considered for this high-risk group.