Caries prevalence and several caries-related factors were measured in 158 pre-school children in Reykjavik, initially aged 4 years and followed longitudinally for 2 years. The mean dmfs at 4 years was 3.3 and rose to 5.8 at 6 years although 34% remained caries-free throughout the study. Caries was very unevenly distributed. Children with dmfs ≥ 5 at 4 years were significantly more likely to have caries in their permanent dentition at 6 years than those with a lower caries score at 4 years. Almost all children carried Streptococcus mutans but only 58% carried lactobacilli at 4 years. Lactobacilli declined in numbers at 5 and 6 years as the number of open carious lesions decreased but the mean count of S. mutans remained fairly static ( > 105 cfu/ml). There were strong associations between high counts of S. mutans or lactobacilli and caries. Salivary parameters were not as strongly associated with caries although a low salivary pH and low salivary flow rate were significantly associated with high caries scores. The misuse of sugar as determined by dietary questionnaire was strongly associated with a high caries score and low caries prevalence was found in those children that took fluoride tablets regularly.