The objective of this study was to assess trends in dental caries prevalence and severity in 1- to 4 year-old children living in Diadema, Brazil, over a 11-year period, from 1997 to 2008. In 2008 an epidemiological oral health survey was carried out and the results on caries were compared with five cross-sectional studies carried out using the same methods and criteria in 1997, 1999, 2002, 2004, and 2006 in the same city. In all surveys, children were randomly selected from those attending a National Day of Children's Vaccination. Calibrated dentists carried out the clinical examination using WHO criteria. Caries trends were assessed by time-lag analysis. In total, 5348 children were examined in the six surveys over the 11-year period. Time-lag analysis showed a marked and statistically significant decline in the prevalence (chi(2) for trends: P < 0.001) and severity (Kruskal-Wallis: P < 0.001) of dental caries between 1997 and 2008. In conclusion, the last cohort of preschool children in Diadema had much better dental caries status than those in 1997.