39 children were studied longitudinally at the age of 2, 3, and 4 yr for the colonization of S. mutans in plaque and saliva and for caries experience. S. mutans was found in 38% of the children, and the predominant serotype group was c/e/f. A total of 16 children got caries before the age of 4. Children who harbored S. mutans in their plaque at the age of 2, appeared to be the most caries-active individuals. Their caries index values (number of decayed, missed and filled surfaces, dmfs = 10.6 +/- 5.3) at the age of 4 differed significantly from the values of children who harbored S. mutans later (dmfs = 3.4 +/- 1.8, P less than 0.005) or remained free from S. mutans infection (dmfs = 0.3 +/- 1.1, P less than 0.0003). It was thus concluded that the early establishment of S. mutans in the plaque of primary incisors indicated early and extensive caries attack in young primary dentition.