This study evaluates the process of implementation of a longitudinal intervention program to promote oral health in preschool children in Flanders, Belgium. As the program was implemented in an existing preventive health care organization, the study also evaluates this setting as the context for implementation. Qualitative and quantitative methods were used to evaluate implementation fidelity, based on Carroll’s theoretical framework of implementation fidelity (Carroll et al., 2007). Questionnaire data from participants and health workers were analyzed and document analyses were performed to compare registrations of the actions with the planning manual. Results were mixed. Whereas more than 88% of all parents attended all home visits, only 57% received at least 9 of the 11 planned consultations. Fifty-two percent of the families received all supporting materials and on average 73% of all attending families received all information at a contact as described in the manual. Moderating factors such as the adequate use of facilitators and high participant responsiveness had a positive impact on implementation fidelity, whereas the quality of delivery differed to a great extent between the nurses who were involved during the entire intervention period and those who gave only a few sessions. Implementing an intervention in an existing well-baby program has many advantages, although lack of time presents a challenge to implementation fidelity. The results of this process evaluation allow a better understanding of the contribution of implementation fidelity to the effectiveness of health promotion programs.