In a long-term study of over 20,000 children aged 6–15 (permanent dentition) and over 12,000 children aged 3–8 years (deciduous dentition), the caries prevalence (DMFT and dft indices) was shown to depend directly upon maintaining water fluoridation at a constant level of 1.0 ± 0.1 ppm F. For a period of 12 years, following the introduction of water fluoridation in Karl-Marx-Stadt in 1959, there was a reduction in caries incidence followed by a slight increase when, for technical reasons, the fluoride concentration was suboptimal for several years. Once fluoridation to 1.0 ppm was reinstated, this slight increase in caries incidence disappeared. Only after 18 years of water fluoridation, however, was the situation thoroughly restored and the observations made in the present paper stress the need for precise technological control and for a constant monitoring of fluoride concentrations in the drinking w