Until 1977, the incidence of lung disease due to Mycobacterium kansasii in Japan was as low as 0.11 or fewer per 10(5) people per year. It began to increase from 1978, reaching 0.33 per 10(5) people per year in 1981. In contrast, the incidence of lung disease due to the M avium-M intracellulare complex was 1.22 to 1.69 per 10(5) people per year from 1977 to 1981. In 1977, the fraction of the disease due to M kansasii in all atypical mycobacterioses was 6.9 percent, while in 1981 it increased to 19.5 percent. Concurrent with this increase, the disease spread all over west Japan, although until 1977 the disease had been restricted almost entirely to Tokyo and its environs. This change of the epidemiologic state of atypical mycobacteriosis seems to be related to the decrease of the incidence of lung tuberculosis.