In 50 patients from 41 families of Duchenne muscular dystrophy (DMD) born between 1956 and 1980 in the San-in area of Japan, the changes in DMD incidence rate were analyzed through five periods of time: 1956–1960, 1961–1965, 1966–1970, 1971–1975 and 1976–1980. The overall incidence decreased from 22.31 X 10^–5 live male births in the first period to 14.51 X 10^–5 in the last. This decrease was mainly related to the group of mothers who were definite carriers (p < 0.005), and was probably due to genetic counselling of definite carrier families. For a further decrease in the incidence of DMD, the newborn male screening system for creatine phosphokinase (CPK) and appropriate genetic counselling might be effective. From an analysis of 28 families without previous family history, the expected proportion of maternal carriers was 0.57.

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