Province de Luxembourg in Belgium is an area with clustering of obesity as well as other risk factors for cardiovascular diseases and non-insulin dependent diabetes mellitus. In this study, obesity was studied in a total of 1028 boys and girls in age-strata 6-8, 8-10, 10-12 years, selected at random from school classes. Participation rate was about 70%. Non-participants did not differ from participants in comparisons of school records of height and weight. Furthermore, information on birth weight, parents' height and weight was collected, as well as reported problems of obesity in grandparents. The results show a high prevalence of pronounced obesity in these children. BMIs were strongly correlated between the children and both parents. Furthermore, grandparents' obesity problems were related to the BMI of parents, and also to obesity indices in the children. Birth weights were related to current BMI of the children (in girls only for the youngest age-group), and to their mothers' BMI. It was concluded that obesity is prevalent in this area and can be traced through three generations, and seems to be discoverable already at birth. Energy intake is high and physical activity level is low in these children. However, statistical analyses suggest that familial factors exert a greater impact than environmental factors on the BMI of the children. These results, as well as the wide-spread, consistent familial clustering of obesity, traceable already at birth, suggest influence of strong genetic traits for obesity in this population.