Fifteen hundred seventy-one leprosy patients registered in the leprosy Mission Hospital from Mar. 1957 to Sep. 1980 were investigated for dapsone resistance by clinical and bacteriological aspects. The results are as follows: There were 29 suspected dapsone-resistant cases out of a total 1571 registered leprosy patients (1. 85%), 26 of 650 of lepromatous type(4. 00%), and 3 of 164 of borderline type (l. 83%). In 1969, we observed the first case if suspected dapsone-resistant leprosy. Since then, suspected dapsone-resistant cases increased and we observed as many as 8 cases in 1977. But frequency of cases has decreased since 1978. (countinued..)