The prevalence of dental caries and dental fluorosis was examined in 380 14-yr-old children living in four geographic areas of Sri Lanka with water F-levels of 0.09-8.0 ppm. A reduction in caries prevalence by 43% was recorded in children consuming 0.6-0.79 ppm F- compared to those in low fluoride areas (< 0.4 ppm). Among those consuming drinking water containing < 1.0 ppm F-, however, 32% of the children had mild forms and 9% severe forms of dental fluorosis (Dean's index). Although other sources of F- may contribute to this effect, the prevalence and severity of dental fluorosis seen in low fluoride areas was confirmed to be high in rural Sri Lanka. Our data are comparable with recent findings from other tropical countries, e.g. Kenya and Senegal, and reaffirm that WHO guidelines for the upper limit of F- in drinking water may be unsuitable for developing countries with a hot, dry climate. Current knowledge now enables us to recommend 0.8 ppm as an appropriate upper limit for F- in drinking water supplies for these populations.