OBJECTIVE To assess the anti-erosion effects of a 0.454% stannous fluoride dentifrice versus a marketed dentifrice in an in situ clinical study. METHODS This was a double-blind, randomized and controlled, two-treatment, four-period crossover clinical study involving healthy adults. Each study period was 10 days. Subjects were randomized to one of two dentifrice products each period: an experimental 0.454% stannous fluoride dentifrice (1,100 ppm fluoride) or a marketed 1.5% arginine-containing dentifrice (Colgate Maximum Cavity Protection, 1,450 ppm fluoride). Subjects wore an intra-oral appliance fitted with two polished human enamel samples for 6 hours per day, swishing with the assigned dentifrice slurry twice a day in addition to sipping and swishing with 250 ml of orange juice for 10 minutes (in increments of 25 ml each minute) four times each day. Contact profilometry was used to measure surface loss of tooth enamel over the course of the study. Two measurements for each sample were taken at baseline and Day 10. RESULTS 35 subjects were randomized to treatment and 31 completed the study (mean age = 40 years). At Day 10, enamel loss means were 0.128 µm for the stannous fluoride dentifrice and 1.377 µm for the arginine-containing dentifrice, respectively (P< 0.001). This represents 90.7% less enamel loss for the stannous fluoride dentifrice. Both products were well tolerated. CONCLUSIONS The 0.454% stannous fluoride dentifrice demonstrated significantly greater protection to human enamel against erosive acid challenges relative to the marketed 1.5% arginine-containing dentifrice in this in situ clinical study.