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For all the 121 municipalities in Southern Norway where 60% or more of the inhabitants get water from registered water supplies, the municipal average concentration of natural fluoride in the water has been determined and the municipalities divided into three groups with fluoride level 0--0.05 mg/1, 0.06--0.10 mg/1, and 0.11--0.50 mg/1, respectively. The average, age-adjusted municipal mortality rate of cancer in the mouth and throat was then determined for each of these groups by sex. The resulting figures show that the mortality rate declines for both sexes with increasing fluoride level: For males from 4.7 deaths per 100 000 per year on the low fluoride level, to 3.9 deaths per 100 000 per year on the medium fluoride level, and to 3.1 deaths per 100 000 per year on the high fluoride level. For females the corresponding figures are 1.5, 1.3 and 0.5. Arguments to show that these figures are unbiased from different tobacco consumption habits, differences between urban and rural municipalities, and differences in population size are presented. Possible, explicatory models are discussed.