AIMS: To assess the prevalence of obesity and changes in body mass index (BMI) distribution between 1989 and 1999 in 5-y-old children, and to study the influence of parental socioeconomic status on these parameters. METHODS: Two cohorts of children in the final year of nursery school (in the city of Lille, France) were enrolled in 1989 (705 children: mean age=5.6+/-0.4 y) and 1999 (1258 children: mean age=5.6+/-0.5 y). Weight and height were measured, and data about parental occupation were collected during a school medical examination. International Obesity Task Force cutoff points were used to define overweight and obesity. Parental occupation was classified into four categories. RESULTS: The prevalence of obesity increased from 1.8 to 4.9%, and the prevalence of overweight rose from 9.6 to 16.9%. Mean-difference plots allowed qualitative comparisons of the BMI distribution between the surveys: for children from the highest social classes, there was no change in BMI; for children from intermediate classes, there was a up-shift only in the upper part of the distribution with the heaviest children becoming heavier still; finally, for children from the lowest class, there was an increase in BMI across the entire population. CONCLUSIONS: This study of the changes in BMI distribution gives greater insight into the 'obesity epidemic'. Our results show the influence of an interaction between genetic and environmental factors. The increase in BMI in the upper part of the distribution suggests that this is a population with a high degree of susceptibility, whereas the increase in BMI across the whole population in the lowest social class suggests a strong influence of the environment on this group and thus the necessity of appropriate, preventive measures.