On account of the recent increases in prevalence of childhood obesity in Western countries, the present study tried to verify a secular trend for increasing body mass index (BMI; kg/m2) in preschool children in Aachen, Germany. The total sample was based on weight and height data for all 99,500 children of German nationality before enrollment in school in the City of Aachen from 1968-1999. For each year, 10% of the boys and girls respectively, were randomly selected for the analyses. Quantile regression was used to examine the pattern and extent of change in BMI percentiles over this 31-year period. Anthropometric data of a total of 5081 boys and 4863 girls were subjected to quantile regression. While significant increases occurred for any given BMI percentile, the annual increase for both sexes was most prominent in the upper range. No change in body height was observed during the study period. Conclusion: preschool children have gained a higher body mass index during the last 30 years. The mechanisms underlying the secular trend towards increasing body mass index seemingly affect children in the upper weight range more than those in the lower range.