In the late seventies an increasing number of children showing extensive and severe idiopathic hypomineralization of the enamel of incisors and permanent first molars was reported within the Public Dental Services in Sweden. An epidemiologic study was initiated to analyze the prevalence, extension and severity in Swedish children born in 1970 and in the years before and after. 2252 children born in 1966-74 were examined according to well defined criteria on enamel hypomineralization. It was found that 15.4% of the children born in 1970 showed such changes. The corresponding figures for children born in 1966, 1969, 1971, 1972 and 1974 were 6.3, 7.3, 7.1, 5.2 and 4.4, respectively.