A cross-sectional survey was undertaken in 1987 to measure the difference in caries experience in schoolchildren between a fluoridated (0.8 mg/L F-) and a non-fluoridated region (0.1-0.4 mg/L F-). 3436 children aged 5-15 yr were examined. Children in the non-fluoridated region had a higher caries experience than those in the fluoridated region. Mean differences were 1.06 dfs (95% CI = 0.66 to 1.47, P less than 0.001) and 0.48 DFS (95% CI = 0.23 to 0.72, P less than 0.001). After adjusting for potential confounding factors (fluoride tablet consumption, socioeconomic status, number of fissure-sealed surfaces, and mobility between regions) the relative risk of not being caries-free in the non-fluoridated region compared with the fluoridated region was 1.43 (95% CI = 1.21-1.70, P less than 0.0001) for the primary dentition and 1.39 (95% CI = 1.18-1.63, P less than 0.0001) for the permanent dentition.