In Japan, the decline of the tuberculosis incidence rates has been slowed down since 1970s. To study factors influencing this slow down of the decline, we have carried out the analysis of RFLP patterns of 941 strains of M. tuberculosis which were isolated at 38 hospitals in various districts of Japan in 1992. The outline of the results is as follows; (1) Distribution of the number of IS6110: The number of the occurring IS elements varies from 1 to 19, and the majority of the isolates have 9 to 14 copies. This finding is identical to the result of the previous investigation carried out in 1987 using 123 strains of M. tuberculosis. There were groups of individuals with identical patterns among those having the same number of copies. The characteristics of the RFLP pattern variety in Japan looks like that of Africa where tuberculosis in highly prevalent. In our country, however, it is considered that the influence of elderly people is very important. Thirty-seven strains contained only one hybridizing band. In 35 of these strains the copy was observed in a 7.9 kbp fragment, and in the other two strains the copy was observed in a 1.5 kbp fragment. Isolates which contain only one or small number of copies could not be differentiated by IS6110, so that other targets for RFLP analysis such as IS1081, DR sequence, and PGRS are to be further investigated. (2) Cluster analysis was shown to be an appropriate epidemiological methodology.(ABSTRACT TRUNCATED AT 250 WORDS)