Clinical examinations of 276 5-year-old children were carried out twice yearly for 2 1/2 years, and a further 1104 11--12-year-old children were examined annually for 3 years. Fifty per cent of permanent first molar teeth had erupted by 6.4 years; the mean age of eruption for permanent second molar teeth was 12.2 years. One year after these mean eruption dates over 10% of first molars and 45% of second permanent molars were already carious. Although development of caries was initially slower in permanent first molars, between the ages of 7.5 and 8.5 years the caries increment rose to 18.8%. By the age of 15 years the DMF was 92% for first and 68% for second molar teeth. The missing and filled proportion of the total DMF rose consistently. This represents the amount of treated caries and exceeded the amount of untreated caries in permanent first molars at all age levels. The rate of caries initiation in the present investigation emphasizes that restorative programmes and preventive regimens for children must be based on frequent recall examinations of not more than 6-monthly intervals.