ObjectivesTo investigate the effectiveness of hands-on training in parental toothbrushing, with or without semi-annual applications of 5% sodium fluoride varnish in preventing ECC. MethodsStudy was conducted in Hong Kong where water is optimally fluoridated. Children aged 8–23 months were recruited and randomly allocated to one of three groups: Gp 1 – control, one-off oral health education talk to parents; Gp 2 – oral health education talk and parental toothbrushing training, reinforced every 6 months; Gp 3 – semi-annual application of fluoride varnish onto child's teeth in addition to the intervention provided to Gp 2. Clinical examinations of the children and interviews were conducted at baseline and after 24 months to assess the children's dental caries status and toothbrushing behaviour. ResultsOut of the 450 child–parent dyads recruited at baseline, 415 (92%) remained after 24 months. At baseline, 2% of the children had non-cavitated enamel caries lesions and the mean dmft score was 0.03±0.24. Most of the children did not have daily parental toothbrushing (65–73%) and self toothbrushing (86–90%). At 24-month follow-up, including both non-cavitated and cavitated carious lesions, the incidences of ECC in Gp 1 to Gp 3 were 11.9%, 11.8%, and 17.5%, respectively (p>0.05); and the mean new dmft scores in Gp 1 to Gp 3 were 0.3, 0.2, and 0.3, respectively (p>0.05). Proportions of parents who practiced parental toothbrushing twice daily were 62.7%, 60.4%, and 65.7% in Gp 1 to Gp 3, respectively (p>0.05). ConclusionsIn a water fluoridated area, hands-on training in parental toothbrushing, with or without semi-annual application of 5% sodium fluoride varnish may not have additional effect on preventing ECC in young children with low risk of dental caries compared to provision of oral health education to parents. Clinical significanceIn a water fluoridated area, provision of individual oral health education to parents may be sufficient for preventing ECC in young children below age 3. Supplemental training in parental toothbrushing and semi-annual applications of fluoride varnish may not have additional caries prevention effect in young children with low risk of dental caries.