The oral health-related beliefs of parents have an important impact on the oral health status of their children; however, they are not stable over time. This study aimed to assess the changes, over time, in the determinants of parental oral health-related behaviour based on the Theory of Planned Behaviour and to investigate socio-economic inequalities. The cohort consisted of the parents - mainly the mothers - of 1,057 children born in 2003 and 2004 in Flanders (Belgium). According to the Theory of Planned Behaviour, validated questionnaires, completed at children's birth and at age 3 and 5 yr, assessed parental attitudes, social norms, perceived behavioural control, and intention towards three behaviours: dietary habits, oral hygiene habits, and dental attendance. Linear mixed-model analyses were applied. Positive parental attitudes towards oral health-related behaviours increased between birth and 3 yr of age, whereas the scores for subjective norms and intentions decreased. Scores remained stable for children between three and 5 yr of age. Highly educated mothers had significantly higher scores for attitudes, perceived behavioural control, and intentions than less-educated mothers. Health promotion campaigns should take these natural changes and inequalities of dental beliefs into account when developing and evaluating interventions.