Two cross-sectional studies were performed on probabilistic samples in the Hérault "département"--respectively in 1987-1988 on children born in 1983 and in 1992-1993 on children born in 1988--to measure the evolution of the prevalence of obesity. A greater proportion of high weight for height (Z score of 1.0 and more, international reference population) has been observed in comparison with the first year. A major increase in the prevalence of obesity (setting a Z score of 2 as limit) has been recorded between the two periods (odds ratio = 2.51 (1.57 to 4.02)). It cannot be explained by structural changes of the population as regards factors known to be associated with obesity (urbanization, mother's occupation, family composition, social status, unemployment, distribution according to the country of origin). The possible influence of changes in the way of life is discussed in relation with the literature. This first representative sample study confirms--at least for the district studied--the findings in various populations, suggesting that the prevalence of obesity is increasing in France. The causes of this phenomenon remain to be defined.