Concern at the high prevalence of dental caries in Iceland prompted this survey of 158 preschool children in Reykjavik. On initial examination in 1987 45.8% of the children were caries-free and the prevalence of caries was a mean dmft 2.4 and a mean dmfs 3.3. A dietary questionnaire aimed at discovering the frequency of sugar consumption per week revealed a threshold value of 30 instances of sugar intake per week above which caries prevalence increased markedly. Some of this sugar is consumed in paediatric medicines, particularly antibiotics and anti-asthmatics, which are widely used in Iceland. Caries prevalence (dmft) for children taking such medicines regularly was 3.0 compared with 2.1 for remaining children (P less than 0.05). Those children who took fluoride tablets regularly had less caries (mean dmft 1.1) than those who used fluoride tablets irregularly or not at all (mean dmft 2.8; P less than 0.001). Samples of saliva from the dorsum of the tongue were taken for determination of numbers of Streptococcus mutans and lactobacilli. High bacterial counts were strongly associated with caries. Only 5% of children with more than 5 x 10(5 S. mutans cfu per ml were caries free but 27% of subjects had 67% of the total amount of caries for the group and all of these would have been detected by the bacterial test.