The development of dental caries from the age of 2.5 to 3.5 years was studied longitudinally in 692 children living in the southern suburbs of Stockholm. The parents answered a structured questionnaire concerning the family's social and immigrant background as well as the dietary habits, oral hygiene and fluoride exposure of their children. Furthermore, the occurrence of mutans streptococci and lactobacilli was determined in samples taken from the tongue of the children, and the buffer capacity of the saliva was measured. At baseline examination, 11.3% of the children exhibited dental caries. At follow-up, 1 year later, decayed and/or filled surfaces were registered in 36.7% of the subjects. The majority of the new lesions were located on the occlusal surfaces of the second molar. Ninety-two percent of the children with caries at baseline developed new carious lesions during the 1-year period, compared to 29% of the children who were caries-free at baseline (p < 0.001). Of the lesions diagnosed at baseline as initial caries, 64% progressed to manifest lesions during the 1-year period. The study indicates that children with early caries development exhibit high caries progression as well as a high risk for further development of an extensive number of new carious lesions.