Studies reporting clinical and non-clinical parameters of participants and non-participants of clinical trials are scarce. In the available studies non-participants were likely to show less favourable outcomes than participants on both socioeconomic parameters as well as on caries experience. However, the impact of non-participation on the total sample of the research population is not established. In the present study, as part of baseline data collection for a randomized controlled trial on caries-preventive strategies, 346 parents of children 6.0 years (± 3 months) of age were approached to let their child participate. Sixty parents refused, but 56 of them were willing to fill out the same set of questionnaires and to allow their child to be clinically examined once. Parents from participating children had higher socioeconomic status, were more often of autochthonous origin and scored better on knowledge questions than parents of non-participating children. Furthermore, parents of participating children reported a higher willingness to invest, were more likely to hold on to regular meals and their child had lower levels of plaque compared to non-participating children. Surprisingly, the participating children had higher dmfs scores than the non-participating children. Their care index (fs/ds + fs) was higher than that of non-participating children. Based on the findings of this study, the presumption that non-participating children will show less favourable clinical outcomes cannot be supported. Although participants differed from non-participants, they did not differ from the total population. It is suggested that the external validity of a randomized controlled trial on caries-preventive strategies is not necessarily affected by non-participation bias.