ABSTRACTThe prevalence of plaque and gingivitis in 154 children 11–13 years of age was assessed at intervals of 2 and 6 months after an initial examination. The children were divided into four groups, each receiving weekly mouth rinses with fluoride or a placebo solution. Two of the groups carried out supervised toothbrushing immediately before the rinses. Judged from 48 Pl I and G I scores in each child, the baseline data indicated no sex differences, but slight differences in moderate gingivitis, i.e. score 2, between the groups. Paired comparisons of the score 2 frequencies during the experiment indicated no effect on the incidence of gingivitis caused by the fluoride treatments or the supervised toothbrushing. Thus, the weekly fluoride rinses commonly used in the school dental service do not appear to have a detrimental effect on the gingiva of children. The weekly supervised brushing instruction used in the present program was not able to improve significantly the level of oral hygiene in succeeding period