Oral diseases such as dental caries, periodontal disease, and tooth loss are a considerable public health problem. A review of the epidemiological data from many countries indicates that a global increase in dental caries prevalence affects children as well as adults. Despite the improvement in oral health of children in the last few decades, tooth decay remains one of the most common childhood diseases in both industrialized and developing countries. The study evaluates the effects of a self-regulatory intervention to increase dental flossing among adolescents and examines the mediating mechanisms underlying behavioral changes. A cluster randomized controlled trial compared a brief intervention arm with a control arm in 166 girls aged 11-15years. Planning, self-efficacy, and behavioral intention were specified as mediators between treatment conditions and follow-up dental flossing frequency. At baseline, the intervention group received theory-guided materials on oral hygiene. Four weeks later, changes in behavior and social-cognitive variables were assessed. The brief self-regulatory intervention led to an increase in dental flossing and social-cognitive constructs. A sequential mediator model was identified in which first changes in intention and afterwards changes in self-efficacy mediated between treatment conditions and behavioral outcomes. Intention formation and self-efficacy seem to play an instrumental role in the mechanism that facilitates dental flossing among adolescent girls. Oral self-care interventions should consider the application of intention formation strategies combined with building confidence in ones ability to adhere to the regimen.