We evaluated sputum quality to establish a diagnostic and treatment strategy for Mycobacterium avium-intracellulare complex lung disease (MAC-LD). Of all 905 MAC-LD patients, 564 (62.3%) had nonpurulent sputum. Furthermore, 88 treated patients were associated with purulent sputum. Therefore, sputum quality may be useful for diagnosing MAC-LD and deciding treatment initiation.