The present study was designed to investigate the pattern of the caries inhibiting effect of school fluoride mouthrinse programs in children of different baseline caries experience. In Phase I, 170 children 10 and 11 years of age were randomly assigned to a test group who rinsed with a 0.5% NaF solution once every 2 weeks for 3 years or to a control group who rinsed with distilled water. In Phase II, the dental records of 345 children were used. Here, the test group had rinsed weekly with a 0.2% NaF solution, while the control group consisted of children 3 years older who were not included in the program during the ages under study (10-11 to 13-14). The statistical analysis of the data was performed as linear regression of caries increments on baseline scores. The analyses included all tooth surfaces, as well as separate studies of proximal and free smooth surfaces, respectively. It was shown that caries increments increased as the baseline scores increased. The slope of the regression lines indicated that the fluoride mouthrinses were more effective in high caries groups than in low caries groups. The results are in agreement with earlier findings concerning the interaction between factors in the caries process.