A total of 101 strains of Mycobacterium avium complex (MAC), consisting of 86 M. avium and 15 M. intracellulare strains, were examined by DNA sequencing. The frequency of radiological types [cavitary (Cav) type and nodular bronchiectatic (NB) type] was similar in each species, thus, both species are equal causes of the two radiological types of MAC disease. We also examined serovars of the isolated strains using extracted glycopeptidelipid antigens and thin layer chromatography. Cav type patients discharged a single serovar MAC, while NB type patients discharged two serovar MACs simultaneously. RFLP pattern obtained by the use of IS1245, revealed no clustering of the strains specific for Cav type and NB type. Histopathological examinations of the bronchial lesions in 40 MAC cases and 49 multi-drug resistant tuberculosis (MDR-TB) cases were performed on the surgically removed lung specimens. Lymphocytic infiltration, epithelioid cell granuloma formation, epithelial desquamation and ulceration, as well as smooth muscle atrophy as a cause of bronchiolectasis, were observed more often and were more severe in the peripheral bronchial walls of the NB type than the Cav type of MAC, and as compared with those of MDR-TB cases.