A multicenter, double-blind, randomized, placebo-controlled clinical trial was conducted to determine the efficacy of an experimental dentifrice on the prevention of recurrent aphthous stomatitis (RAS). A 3-month pretrial period was used to self-record data pertaining to the frequency, ulcer size, duration, and pain associated with ulcers. Thirty-six participants, who reported at least 3 RAS episodes or 30 days with RAS during the pretrial period, completed the study. Following a stratified randomization for age, sex and disease severity, the participants received the experimental dentifrice (T) or the placebo (C). The test period comprised 3 months of self-evaluation of number of ulcers, size and location. Pain related to RAS was estimated by the use of a Visual Analogue Scale (VAS). Symptoms were found to decrease in both groups when the pretrial and test periods were compared, although no statistical difference was reached in the C group. A statistically significant difference between the two time periods was obtained for the T group concerning the number of days with ulcers (p 0.025) and VAS (p 0.010). Of more clinical importance was the observation that 50% of the patients in the T group reported a more than 50% reduction in days with ulcers compared to 17% in the C group. The dentifrice may be used as a treatment strategy for a subgroup of patients with RAS in view of the low adverse effects and the benefit of using a treatment modality that is a part of daily routines. Keywords: aphthae, RAS, dentifrice, prevention, controlled-study