Purpose To explore breastfeeding from 6 to 18 months of age and to study the association between breastfeeding and caries prevalence at 5 years of age. Methods The study included 1088 children from one Norwegian county and was based on the Norwegian Mother, Father and Child Cohort Study (MoBa). The children had clinical dental examination at 5 years of age, and parents answered a questionnaire, which included information on breastfeeding, oral health behaviour and child characteristic. Multivariate logistic regressions were performed. The study was ethically approved. Results Of the studied children, 77% were breastfed at 6 months of age and 16% were still breastfed at 18 months of age. Few children (6%) were breastfed during night at 18 months of age, while 11% received sugary drink during night. No association was found between breastfeeding up to 18 months of age and caries prevalence at 5 years of age (p > .05). Children who at 18 months of age had their teeth brushed less than twice daily (OR 2.4, CI 1.5–3.9), consumed sugary drink once a week or more often (OR 1.7, CI 1.1–2.7) and had non-Western parents (OR 3.4, CI 1.5–8.1) were more likely to have caries experience at 5 years of age than other children. Conclusion Breastfeeding up to 18 months of age was not associated with caries development during preschool age.