The aim of this study was to assess the modifying effect of oral hygiene, and the use of fluorides on caries increment in children when frequency of meals, intake of sugars and prevalence of lactobacilli and S. mutans in the saliva were known. In 8- and 13-yr-olds it was possible to show that children with a low caries increment (0-2 surfaces) had lower mean values for frequency of meals, daily sucrose consumption and prevalence of lactobacilli and S. mutans in the saliva than children with a high caries increment (greater than or equal to 3 surfaces). There were higher mean values in the low caries-increment groups for toothbrushing frequency, use of fluoridated toothpaste and NaF-mouthrinse. Gingival bleeding index (GBI %) was used as an objective measure of oral hygiene and revealed statistically significant differences between the two groups in both age groups (P less than 0.05, P less than 0.01) with the highest scores recorded in the high caries-increment groups. Using as discriminating variables number of meals/day, daily consumption of sucrose and other sugars and number of lactobacilli and S. mutans in the saliva, 77% of the 8-yr-olds and 70% of the 13-yr-olds were correctly classified with regard to net caries increment. If GBJ %, tooth-brushing frequency, use of fluoridated toothpaste and NaF-mouthrinse were included in the analysis another 4% of the 8-yr-olds and 11% of the 13-yr-olds could be correctly classified.