The course of caries depends on an equilibrium between de- and remineralization of enamel. Epidemiologic studies performed during recent years in Poland have demonstrated a high incidence and severity of caries in children and teenagers. Therefore, the actual program of caries prevention covering the whole population of children and teenagers has not performed to the expectations, possibly because the program protocol does not discriminate between children susceptible and resistant to caries. The aims of this work included: 1. Analysis of tooth groups and surfaces as to their susceptibility to caries; 2. Evaluation of the influence of de- and remineralization modifying factors such as: a) dental plaque and sugars, b) caries-causing bacteria, c) enamel resistance to acids, d) biochemical properties of saliva--on the susceptibility to caries; 3. Evaluation of the uptake of fluorides from chewing gum by enamel in relationship to caries susceptibility. An initial clinical examination of permanent teeth was performed in 292 children aged 12 years. Subsequently, two groups were formed: one with 45 children free from caries and another with 77 children at high risk of caries. Control examinations of permanent teeth in both groups were carried out after one and three years from the initial examination. The clinical examination included a check-up of secondary dentition, counting of erupted teeth, sealed teeth, evaluation of oral hygiene status, gingivae, abnormalities in mineralization and measurement of the rate of microdemineralization. The biochemical analysis of saliva was done to measure the content of fluorides, calcium, phosphorus and magnesium, as well as pH. Microsamples of enamel were collected using acid biopsy and the content of calcium and fluorine in the superficial and underlying layer was assayed. The thickness of both layers obtained by biopsy was also measured. Commercial test kits--Dentocult SM and Dentocult LB, served to determine the amount of caries-causing bacteria in saliva. Additionally, the children and their mothers responded to a questionnaire on the diet, previous diseases, therapies with antibiotics and health habits. The results were analysed using Statgraphics software. Differences between means were checked using Student t-test in its classical form or as modified by B. L. Welch. Linear correlation indices were calculated and Spearman rank correlation test was performed. On the basis of the results obtained the following conclusions were drawn: 1. 12-year old children at high risk of caries need intense dental care due to: a) extensive involvement of masticatory surfaces of molar teeth and contact surfaces of premolar teeth, b) treatment inappropriate to actual needs in this group of children. 2. Effective programs to improve the knowledge and health of children at high risk to caries should be introduced due to the following facts: a) oral hygiene was unsatisfactory in the examined children, b) children consumed large quantities of sweets. 3. Disturbances in the mineralization of enamel and the use of antibiotics in childhood were without influence on the susceptibility of teeth to caries. 4. The content of fluorides, calcium and phosphorus in the saliva of children susceptible to caries could have been below the level necessary to maintain an equilibrium between de- and remineralization of enamel. 5. Susceptibility of teeth to caries could be due to differences in the chemical composition and structure of enamel, as it appears from the content of fluorides and calcium, depth of enamel biopsy and uptake of fluorides from chewing gum. 6. Chewing of fluoride-containing gums is a supportive measure in caries prevention, particularly recommended in children at high risk of caries. 7. Caries prevention should be individualized and matched to caries risk.