In December 1973 water fluoridation was discontinued in Tiel, the Netherlands. Six years later an epidemiological study was started, with the aim to investigate the consequences. In the period from 1979 to 1988 a yearly caries investigation was carried out in 15 year old children born and living in Tiel and children living in the control town of Culemborg, where the drinking water had never been fluoridated. The 15 year old children in 1979-1980 in Tiel had a 18% higher caries experience than the same age group in 1968-1969, to whom fluoridated water had been available from birth onward. In the same period a 28% decrease of the caries was found in Culemborg. In 1979-1980 the DMFS-index in Tiel was still 36% lower than in Culemborg. Between 1979-1980 and 1987-1988 the caries experience decreased in both towns.