Randomised controlled trial.Elders having at least five teeth with exposed roots, no serious medical problems and basic self-care ability (including oral hygiene practices) were randomly allocated into one of four prevention groups. Individualised oral hygiene instruction was provided to each participant, focusing on effective brushing with a manual toothbrush, and use of fluoride toothpaste was recommended. Before applications of the study agents, a piece of gauze was used to clean and dry the teeth. Then water (placebo control), chlorhexidine varnish (Cervitec, Ivoclar Vivadent AG, Schaan, Liechtenstein), sodium fluoride varnish (Duraphat, Pharbil Waltrop GmbH, Waltrop, Germany) or SDF solution (Saforide, Toyo Seiyaku Kasei Co. Ltd., Osaka, Japan) was applied onto the exposed root surfaces of participants in the respective groups by means of a disposable microbrush. The participants were instructed not to eat for half an hour after treatment. Applications of water or SDF solution were repeated every 12 months, and applications of chlorhexidine varnish or sodium fluoride varnish were repeated every three months.Root Caries Index (RCI) was calculated as follows: (no. of root caries lesions/no. of teeth with gingival recession/person) × 100. Treatment effects were also measured by prevented fraction (PF), relative risk and the number (of elders) needed to treat (NNT) to prevent one elder from developing root caries.Two thirds (203/306) of the included elders were followed for three years. Significantly lower relative risks for developing new root caries were found in the elders in the chlorhexidine, sodium fluoride and SDF groups compared with the control (OHI only) group. The mean numbers of new root caries surfaces in the four groups were 2.5, 1.1, 0.9 and 0.7 respectively (ANOVA, p < 0.001). The prevented fraction and numbers needed to treat are shown in Table 1.Applications of SDF solution, sodium fluoride varnish and chlorhexidine varnish are more effective in preventing new root caries than OHI alone. The results of this study provide support for the clinical and community use of the three test materials, in addition to improvement in oral hygiene, to prevent the development of root caries in institutionalised elders.