A supervised brushing study extending over a period of 31 months was carried out to evaluate the anticaries effectiveness of a monofluorophosphate dentifrice with a calcium carbonate abrasive system compared with a fluoride-free placebo dentifrice and a monofluorophosphate dentifrice with an insoluble sodium metophosphate abrasive. The study was of double-blind controlled design and included 712 8- to 12-year-old children living in 2 North Dakota cities, Bismarck and Fargo, the drinking water of which had been fluoridated to 1.2 ppm since 1955. A comparison between the 3 treatment groups demonstrated that the anticaries effect of the calcium carbonate-monofluorophosphate dentifrice was significantly greater than the other two dentifrices. Thus, an additional benefit was observed for the calcium carbonate based dentifrice over and above that assumed to have been due to water-borne fluoride.