STUDY OBJECTIVE--To examine the associations of social and biological factors with measures of obesity in children. DESIGN--The study had a cross sectional design. SETTING--The analyses were based on data from two national study of health and growth cross sectional surveys. The "representative sample" comprised 1990 data from 22 English areas and 1990-91 data from 14 Scottish areas; the "inner city sample" comprised 1991 data from 20 English areas. PARTICIPANTS--The subjects were primary school children aged mainly 5-11 years living in England and Scotland. The "representative" sample included 10,628 children--6463 living in England and 4165 living in Scotland. The "inner city" sample included 7049 children--2183 white, 1124 Afro-Caribbean, 2696 Indian subcontinent, and 1046 from other groups. Due to missing values on continuous variables, 8374 children were included in the analyses. MEASUREMENTS AND MAIN RESULTS--The relation between social environment and childhood overweight was studied using several indicators of obesity. Triceps, subscapular, the sum of triceps and subscapular skinfolds, and weight for height were used as dependent variables. The analyses were carried out in two stages. Firstly, multiple linear regression analyses were used to assess the factors associated with dependent variables treated as continuous. Secondly, multiple linear logistic regression analyses were used to examine the association between independent factors and overweight and fatness defined as binary variables. Birth weight, mother's body mass index (BMI), and father's BMI were consistently associated (p < 0.001) in all models and were the variables that contributed most to the explained variation in the dependent variables. In the multiple regression analyses there was a consistent interaction between the effects of ethnic origin and family size on each outcome variable. In the logistic regression analyses the interaction was not significant, and highly significant associations between both overweight and fatness with the number of children were shown. Ethnic group was not significantly associated with overweight but it was with fatness. The strengths of the remaining significant associations were slight and inconsistent in relation to the dependent variables or the type of analysis. CONCLUSION--Very few variables were associated with measures of overweight and fatness. The only useful factor that was highly associated with all measures of fatness was the parents' BMI. Strategies to prevent childhood obesity should be aimed at the total population and special emphasis should be placed on families in which one or both parents are overweight.