Using Gen-Probe test, both the Mycobacterium avium and Mycobacterium intracellulare strains were identified in the 76 disease-associated Mycobacterium-avium complex (MAC) strains isolated in the NIIGATA area. The following results were obtained: 1) Fifty-four MAC strains (71.1%) reacted with M. avium-probe and 22 strains (28.9%) reacted with M. intracellulare-probe. 2) There were no significant differences between the ratio of two strains before and after 1991. 3) The ratio of female cases with M. avium was significantly higher (22/32) in the later period (after 1991) than in the former period (9/22).