About 85% (n = 269) of the subjects who participated in the Ylivieska follow-up studies on the effect of xylitol chewing gum on dental caries during 1982-84 or 1982-85 were re-examined in 1987 for the analysis of possible long-term preventive effects. Further caries reduction was found 2 or 3 yr after the discontinuation of the use of xylitol. The effect was especially marked in girls; the reduction in caries increment in the post-use years was 60% for the 2-yr users, suggesting that more pronounced caries reduction was associated with the most regular use of xylitol. In teeth erupting during the first year of the use of xylitol gum the long-term preventive effect was greater than in other teeth. Several explanations are suggested: lasting effect of the microbiological changes in the mouth, bacterial colonization on newly erupted teeth by organisms other than S. mutans, and/or thorough maturation of the teeth under favorable physico-chemical circumstances. The results suggest that the value of xylitol in caries prevention depends on the timing of the treatment in relation to the development of the dentition.