OBJECTIVE Little is known about the effect of Cervitec, a chlorhexidine-thymol varnish, on root caries. Our objective was to determine whether a 3-monthly application of Cervitec over 1 year would limit the progress of existing root caries lesions and reduce the incidence of dental root caries in a group of dentate institutionalized elderly, as a complement to their usual oral hygiene practices. METHODS A double-blind randomized clinical trial was conducted in 68 subjects (34 per group) in two residences in Almería (Spain). Twenty-one subjects with 60 root caries lesions and 25 with 65 lesions, in the Cervitec and placebo groups, respectively, completed the study. Varnishes were applied twice in the first week, 1 month later, and every 3 months until the end of the study. Clinical parameters associated with established lesions were determined at baseline and after 6 and 12 months, as was the incidence of root caries lesions. RESULTS The clinical evolution of lesions was significantly better in the Cervitec group as opposed to the placebo group in terms of width, height, color, and texture. The increase in root caries was significantly lower (p=0.039) in the Cervitec group. CONCLUSIONS According to these results, Cervitec may help to control established root lesions and reduce the incidence of root caries lesion among institutionalized elderly.