Objective: This study aimed to test the predictive validity of the Theory of Planned Behaviour (TPB) when applied to the oral health-related behaviours of parents towards their preschool children in a cross-sectional and prospective design over a 5-year interval. Methods: Data for this study were obtained from parents of 1,057 children born between October 2003 and July 2004 in two regions in Flanders, Belgium. Three behaviours related to oral health (dietary habits, oral hygiene habits, dental attendance) as well as their psychological determinants based on the TPB (attitude, subjective norms, perceived behavioural control (PBC), intention) were measured using validated self-report questionnaires when the children were 0 (2003-2004), 3 (February-June 2007) and 5 years old (March-June 2009). Results: Multiple linear regression analyses indicated cross-sectionally that the contribution of attitudes, subjective norms and PBC towards intention, and of intention and PBC towards the frequency of consumption of sugared snacks and drinks, tooth brushing and dental attendance was significant (p < 0.001). The combined determinants explained 27%-37% of the variance in intentions and 7%-39% of the variance in the behaviours. Across time, only parental PBC as measured at birth could significantly predict dietary and oral hygiene behaviours at children's ages 3 and 5 years. Conclusion: This study supports the validity of the TPB to predict intentions and oral health-related behaviours of parents of preschool children. Interventions should aim to improve parental attitudes towards diet and dental attendance, and their PBC towards tooth brushing.