The caries increment (filled surfaces) from the ages 7 to 15 years were compared in children with three or fewer (low prevalence group) or eight or more filled surfaces (high prevalence group) at the age of 8. The children participated in a fortnightly fluoride mouthrinsing program (10 ml of 0.2% NaF). Following 8 years of dental treatment and caries prophylaxis, the caries increments were 11.4 (s.d. = 7.7, n = 23 subjects) and 17.1 (s.d. = 9.6, n = 39 subjects) surfaces, i.e. significantly different (t = 2.376). Significantly (t = 4.034) more fillings had been required in the high than in the low prevalence group (31.1 +/- 17.1 vs. 15.5 +/- 9.6). The "risk group" could be identified at the ages of 7 to 8 by high caries prevalence and high ratio fillings/caries increment. Social class and number of teeth accounted more for the initial caries prevalence than for the caries increment. Correlation analyses revealed a significant, but not strong (r = 0.50), association between caries prevalence at the age of 7 and increment of fillings.

The fluoride ion activities of two NaF dentrifrices have been determined in undiluted samples, in samples diluted in 6 ml water or saliva, and in saliva-dentifrice slurries after 2 min brushing. The activity potentials were compared with the activities of NaF standards. The fluoride ion concentration of each dentifrice was determined when 0.1 g dentifrice was dissolved in 5 ml water and 20 ml buffer. The activities in diluted samples were about twice the activities in in vivo samples. The fluoride ion activity in saliva before fluoride application was below 0.06 ppm and rose to about 1 ppm half an hour after fluoride application.

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