BACKGROUND The effect of the fluoride in the caries prevention is strongly linked to its topical action. When the fluoride is ingested, its anticaries effect is weak and, in addition, it can cause side effects such as dental fluorosis in children. The toothbrushing with fluoride dentifrice is a suitable procedure to provide the topical fluoride, but in small children there is risk of inadvertent swallowing. The main goal of this study is to quantify the fluoride levels in urine after toothbrushing with fluoride dentifrice. METHODS A sample of 115 children between 5 to 8 years was randomly selected. The pH, as well as creatinine and fluoride levels were analyzed in the urine excreted before and two hours after toothbrushing with fluoride dentifrice. The fluoride/creatinine (F/Cr) ratio was used to minimize the variations of the fluoride levels in urine due to the different rates of secretion. For the statistical analysis, the average, the standard deviation and the T test were used for the variables with normal distributions, and nonparametric tests were applied for the variables with non normal distribution. RESULTS The average amount of dentifrice was 1192 mg (SD: 421). The variables pH and creatinine showed similar values in the two samples of urine analyzed (pH: 6,35 and 6,36; creatinine: 0,76 g/l and 0,81 g/l). Nevertheless, the F/Cr ratio increased significantly after toothbrushing, ranging from 0.61 to 1.25 mg/g. CONCLUSIONS The use of fluoride dentifrice is related to a significant increase of the fluoride concentration in urine in children.