Background: Identifying the early determinants of risk for dental caries would be useful in its primary prevention. Subjects and Methods: All children at the age of 18 months in one health care area were invited in the screening test for mutans streptococci (MS) and their caretakers interviewed for the study purposes. The 366 children who met the inclusion criteria were clinically examined after 42 months. Results: At the age of 5 years ± 6 months, in 79 (21.6%) children the 42-month caries increment was >0. In the multivariate regression analysis, the strongest correlation was found with MS detected in the oral biofilm. Of the reported background factors the blue-collar occupation of the caretaker, child’s sugar use, night feeding, use of other thirst quenchers than water and father’s reported poor oral health were significantly correlated with the child’s caries increment. Conclusion: The oral health of a child is strongly related to the lifestyle and the oral health habits of the caretakers during the first years.