In several human studies, scientists have sought to elucidate the connection between caries activity, high levels of lactobacilli, low buffer capacity and a low salivary flow rate, separately or in combinations. The aims of the study were to investigate if there was a difference in caries activity among adults in optimal versus low fluoride areas. It was further investigated if this difference was related to high lactobacilli levels, low buffer capacity and low salivary flow rate, separately or in combination. Finally, it was analysed if the lactobacilli levels were lower in an area with optimal fluoride water concentration. The study is based on clinical examinations of 30-40 year olds who all their lives had drunk water with a fluoride concentration representative for the community. 260 individuals living in the optimal fluoride area and 236 from the low fluoride area are included in the study. The results show a caries activity significantly lower among those in the optimal fluoride area. It is also shown that these differences can not be explained by differences in lactobacilli levels, buffer capacity and salivary flow, neither separately nor in combination. The fluoride concentration in the drinking water is wholly decisive. No difference in lactobacilli levels between optimal and low fluoride areas were demonstrable in this study.