The rise and fall of caries prevalence (DMFT) and its relation to changing F concentration of drinking water and other health-related factors is analysed based on dental findings of more than 286,000 subjects of either sex (6-15 years old) from the two industrial towns Chemnitz and Plauen. Water fluoridation (1.0 +/- 0.1 ppm F) was implemented in Chemnitz (formerly Karl-Marx-Stadt) in 1959. It was in operation until autumn 1990 with an interruption lasting 22 months around the year 1971. In the F-poor town of comparison, Plauen, 55% of the citizens were supplied with F-enriched drinking water (0.9 ppm F) during the years 1972-1984. Another 20% received F-containing mixed water (0.4-0.7 ppm F). During the first three decades of the study the level of caries prevalence was strictly correlated with the availability of an optimal caries preventive F concentration in the drinking water. Water fluoridation was followed by a decrease of caries, and interruptions in fluoridation were followed by increasing caries levels. A different caries trend was observed in the years from 1987 to 1995. There was a significant caries decrease down to the lowest DMFT (2.0) since 1959 in spite of the fact that only F-poor water was available over years in both towns. This improvement of oral health is explained by changes in caries-preventive and environmental conditions.