Results: After one year, as compared to caries free group, the change of S mutans level in high caries group was 1.25 times more (95% CI: 1.09 - 1.43, p<0.05); the percentage of children in high caries group that acquired at least one new lesion was significantly higher (17.5 times higher in deciduous teeth, 3.08 times higher in permanent teeth, p<0.05).