OBJECTIVE The aim of this single - blind, multicenter, parallel, randomized controlled trial was to evaluate the effectiveness of the application of a high-fluoride toothpaste on root caries in adults. METHODS Adult patients (n = 130, ♂ = 74, ♀ = 56; mean age ± SD: 56.9 ± 12.9) from three participating centers, diagnosed with root caries, were randomly allocated into two groups: Test (n = 64, ♂ = 37, ♀ = 27; lesions = 144; mean age: 59.0 ± 12.1; intervention: high-fluoride toothpaste with 5000 ppm F), and Control (n = 66, ♂ = 37, ♀ = 29; lesions = 160; mean age: 54.8 ± 13.5; intervention: regular-fluoride toothpaste with 1350 ppm F) groups. Clinical examinations and surface hardness scoring of the carious lesions were performed for each subject at specified time intervals (T0 - at baseline before intervention, T1 - at 3 months and T2 - at 6 months after intervention). Mean surface hardness scores (HS) were calculated for each patient. Statistical analyses comprised of two-way analysis of variance and post hoc comparisons using the Bonferroni-Dunn correction. RESULTS At T0 , there was no statistical difference between the two groups with regard to gender (P = 0.0682, unpaired t-test), or age (P = 0.9786, chi-squared test), and for the overall HS (Test group: HS = 3.4 ± 0.61; METHODS HS = 3.4 ± 0.66; P = 0.8757, unpaired t-test). The anova revealed significantly better HS for the test group than for the control groups (T1 : Test group: HS = 2.9 ± 0.67; METHODS HS = 3.1 ± 0.75; T2 : Test group: HS = 2.4 ± 0.81; METHODS HS = 2.8 ± 0.79; P < 0.0001). However, the interaction term time-point*group was not significant. CONCLUSIONS The application of a high-fluoride containing dentifrice (5000 ppm F) in adults, twice daily, significantly improves the surface hardness of otherwise untreated root caries lesions when compared with the use of regular fluoride containing (1350 ppm F) toothpastes.