There are a few recent reports about the relationship between the clinical effect and drug-sensitivity test. We investigated the relationship between the clinical efficacy of treatment for pulmonary Mycobacterium avium complex (MAC) and drug-sensitivity test for isolated MAC by comparison between data from 2005 to 2007 and from 2008 to 2010. We studied 60 patients who satisfied diagnostic criteria of nontuberculous mycobacterial infection established by the American Thoracic Society in 2007 and who received combination therapy using rifampicin (RFP), ethambutol (EB), streptomycin (SM), and clarithromycin (CAM). Average CAM dosage was increased from the early (517 mg/day) to the later (800 mg/day) period. Sputum conversion rate increased from 63% in the early period to 83% in the later period. Clinical improvement also increased from 38% in the early period to 53% in the later period. The causative microorganisms isolated were M. avium in 35 patients and M. intracellulare in 25. In both periods, isolated MAC strains showed excellent minimum inhibitory concentration (MIC) for CAM. Regarding the relationship between clinical efficacy and MICs of RFP, EB, CAM, and SM, most patients with good clinical effects showed low MIC for CAM in both periods. Good clinical efficacy, including the sputum conversion rate, was obtained with an increased dose of CAM in the later period. We speculate that the increased dose of CAM influenced the good clinical effect in both periods.