We encountered two-cases of pulmonary disease caused by M. chelonae subsp. abscessus, [Case 1] A 72-year-old man was admitted to the hospital because of fever. He had been observed for one year after being given a diagnosis of pulmonary disease caused by Myocobacterium avium complex. Sputum examination revealed acid-fast bacilli (Gaffky 9). He recovered after administration of clarithromycin (CAM) and other drugs. [Case 2] A 61-year-old man was admitted to the hospital because of coughing and sputum production. He had been observed for 4 years after being given a diagnosis of pulmonary M. fortuitum disease. Sputum examination revealed acid-fast bacilli (Gaffky 7). His symptoms deteriorated even though he received anti-tuberculosis agents and CAM. After measurement of minimal inhibitory concentration (MIC), he was given amikacin (AMK). In both cases, the bacilli found in sputum obtained on admission were identified as M. chelonae subsp. abscessus by DNA hybridization. They were completely resistant to all anti-tuberculosis agents. However, the disk method show that they were sensitive to AMK, imipenem and CAM. The MIC value of those strains to CAM was 0.78 microgram/ml in case I and more than 100 micrograms/ml in case 2. The results obtained by MIC measurement were consistent with the clinical outcome. AMK, cefoxitin (CFX), and CAM had been used to treat M. chelouae subsp. abscessus in Europe, but the MIC value differed from strain to strain within a species. Thus the present data suggest that measurement of the MIC value of CAM would be necessary to predict its therapeutic effect.