Background: There is a lack of evidence for effective management of dental caries (decay) in children's primary (baby) teeth and an apparent failure of conventional dental restorations (fillings) to prevent dental pain and infection for UK children in Primary Care. UK dental schools' teaching has been based on British Society of Paediatric Dentistry guidance which recommends that caries in primary teeth should be removed and a restoration placed. However, the evidence base for this is limited in volume and quality, and comes from studies conducted in either secondary care or specialist practices. Restorations provided in specialist environments can be effective but the generalisability of this evidence to Primary Care has been questioned.The FiCTION trial addresses the Health Technology Assessment (HTA) Programme's commissioning brief and research question "What is the clinical and cost effectiveness of restoration caries in primary teeth, compared to no treatment?" It compares conventional restorations with an intermediate treatment strategy based on the biological (sealing-in) management of caries and with no restorations. Methods: This is a Primary Care-based multi-centre, three-arm, parallel group, patient-randomised controlled trial. Practitioners are recruiting 1461 children, (3--7 years) with at least one primary molar tooth where caries extends into dentine. Children are randomized and treated according to one of three treatment approaches; conventional caries management with best practice prevention, biological management of caries with best practice prevention or best practice prevention alone.Baseline measures and outcome data (at review/treatment during three year follow-up) are assessed through direct reporting, clinical examination including blinded radiograph assessment, and child/parent questionnaires.The primary outcome measure is the incidence of either pain or infection related to dental caries.Secondary outcomes are; incidence of caries in primary and permanent teeth, patient quality of life, cost-effectiveness, acceptability of treatment strategies to patients and parents and their experiences, and dentists' preferences.Discussion/ Summary: FiCTION will provide evidence for the most clinically-effective and cost-effective approach to managing caries in children's primary teeth in Primary Care. This will support general dental practitioners in treatment decision making for child patients to minimize pain and infection in primary teeth. The trial is currently recruiting patients.Trial registration: Protocol ID: NCTU: ISRCTN77044005 http://www.controlled-trials.com/ISRCTN77044005, http://public.ukcrn.org.uk/search/.