Twenty 17-year-old schoolchildren were selected for the study because of high salivary counts of Streptococcus mutans 106 CFU/ml) and a frequent intake of sugars between meals. After a baseline determination of the numbers of S. mutans and lactobacilli in saliva, and of S. mutans in interdental plaque, the subjects were asked to avoid sugar-containing between-meal products, e. g. sweets, chewing gum, soft drinks and confectionery, and to eat a maximum of 4–5 meals/day. After 6 weeks of sugar restriction, the participants were allowed to return to their former food habits and were then followed for a further 6-week period. Seven subjects, also with high salivary counts of S. mutans and a frequent sugar consumption, served as control group. The results showed a statistically significant decrease of mean salivary S. mutans counts from 1.5 × 106 to 0.3 × 106 CFU/ml (p < 0.001) during the sugar restriction period. The mean number of lactobacilli in saliva decreased from 2.3 × 104 to 0.2 × 104 CFU/ml (p < 0.001). Regarding the interdental plaque samples, the number of tooth surfaces with detectable S. mutans were reduced from 52 to 32% (p < 0.001). When the subjects returned to their former food habits, the S. mutans and lactobacillus values almost reached the baseline level again. No obvious changes were found in the control group for any of the studied variables. To conclude, the 6-week period with sugar restriction reduced the number of S. mutans both in saliva and in interproximal spaces. Also the lactobacillus counts in saliva decr