The effect of a field program, based on systematic plaque control, on caries and gingivitis was tested during a 3-year period on 1,100 schoolchildren. Once every 3rd week the children were given oral hygiene instructions, professional toothcleaning and fluorides topically delivered by specially trained dental nurses. The children of a control group of approximately the same number of pupils, participated in a preventive program consisting of mouthrinsings once every 2nd week with a 0.2% sodium fluoride solution. The children of the third and fourth grades were, at the start of the experiment, selected as reference groups and then continuously examined once every year. At the end of the trial the mean reduction of surfaces haboring plaque and units with gingival inflammation was 59% and 73%, respectively. The reduction in caries increment was 51%.