The purpose of the study was to assess prospectively the influence of social factors in childhood on risk of overweight in young adulthood, while taking into account degree of fatness in childhood. A random selection of children 9-10 years of age attending the third school grade in Copenhagen schools in 1974 was performed. Parental school education, householder's occupational status, and quality of dwellings in the rearing area were recorded. Ten years later a follow-up was carried out, and 756 (86%) of the 881 eligible individuals participated. The effect of the social factors on the risk of overweight in adulthood was analysed by logistic regression analysis, including gender and degree of fatness in childhood as covariates. The relationship between the traditional socio-demographic factors (parental education and occupational) and risk of overweight in young adulthood was insignificant, when controlling for degree of fatness in childhood and gender. However, the risk of overweight was highly increased for individuals reared in an area with poor quality of dwellings compared to those from a good area (P less than 0.001), even when controlling for the effect of parental education and occupation (P less than 0.001). In conclusion, rearing area had a much stronger influence on risk of overweight in young adulthood than parental education and occupation.