Aim: To evaluate the effect of xylitol-containing tablets on mutans streptococci colonisation and caries development in preschool children. Study design: Randomised single-blind prospective design. Methods: The material consisted of 132 healthy 2-year-old children that were assigned to a xylitol tablet (test) group or a non-intervention control group. The test group was given 1-2 xylitol tablets (0.5-1g) per day during 1.5 years. Mutans streptococci enumeration was performed at baseline and semi-annually with a chair-side technique. Caries prevalence was scored at baseline and the age of 4 years. Results: No statistically significant differences in mutans streptococci colonisation were disclosed between the test and control groups at baseline or any of the designated follow-ups. A statistically significant positive relationship was found between the maternal salivary mutans streptococci levels and the colonisation of the children in the control group (r=0.35; p<0.05) but not in the xylitol tablet group. The mean caries prevalence was lower in the test group compared with the control group at 4 years of age (dmfs 0.38 ±1.05 vs. 0.80 ±2.60) but the difference was not statistically significant. Conclusion: The findings do not support a low-dose xylitol tablet program for caries prevention in preschool children.