A longitudinal study on the effect of an individualized preventive program was carried out on 10-11 year old children with higher caries and/or gingivitis prevalence than the average. The program was based on bimonthly professional toothcleaning as well as hygiene and dietary instructions. After an introductory period of 3 months the frequency of visits was individualized according to the gingival condition of the children (every 2nd to 4th week). For one group this program was supplemented by instruction in dental flossing for proximal tooth cleaning on a daily basis. The effects of the preventive measures were evaluated after an experimental 2 year period. Two years after withdrawal of the preventive measures, the longitudinal effects were recorded. In all 627 children were examined and of these 87 children were selected for the study on gingival and cariological indications. It was possible to follow 74 children throughout the 4 year period. After 2 years the number of new caries lesions was significantly reduced in the test groups as compared to the controls. The caries increment in the test group was 1.7 new surfaces per individual and in the control 4.5. The corresponding figures at the follow-up examination was 8.5 and 5.9 respectively. Thus any prolonged effect of the preventive program could not be confirmed. An improved cariological status was also recorded among the children selected on gingival indications. The increment was 0.8 in the test and 3.8 in the control group during the experimental period. The corresponding figures at the supervisory control was 1.8 and 4.6 new surfaces respectively indicating a prolonged effect among these children. The interpretation of the results from the "flossing" group was not clear-cut, since the flossing was performed unsupervised. However, no new decayed tooth surfaces were recorded in those children who had claimed that they had used floss at least every second day. The gingival situation improved in all test groups and was visible even in the follow-up study.