The onset and prevalence of caries in primary teeth and the rate of failure in conservative caries therapy were followed up for six years in a group of children who were three years old at the start of the study in 1974. All children were offered a basic preventive program which included counselling their parents on dietary habits, oral hygiene and the use of fluoride tablets. During the annual clinical examinations bite-wing radiographs were taken in cases with established molar contacts. Operative care was delivered when necessary, combined with topical fluoride applications.At the age of three years the onset of caries was registered in 26 percent of the children. The corresponding percentages at the ages of 4, 5, 6, 7 and 8 years were 26, 18, 8, 3 and 2, respectively. Consequently, 17 Percent were still caries-free at the end of the six-year period.Proximal caries was most often observed on the distal surfaces of the first primary molars, followed by the mesial surfaces of the second primary molars. The overall frequency of class II amalgam restorations requiring replacement was 16 per cent. The highest percentage of failures was noted in lower first primary molars.