Studies of twins and adoptees have shown that the resemblance of adult family members in body mass index (weight/height2) is due more to shared genes than to shared rearing environment. In the present adoption study we assessed the genetic and environmental influences on body mass index in childhood. Among 3651 adult Danish adoptees we selected 840, comprising thin, medium weight, overweight, and obese groups, and for 269 of them we obtained height and weight measured at school health examinations at ages 7-13 years. The correlation in body mass index between the adoptees at these ages and their biological and adoptive relatives as well as among these relatives were estimated. The correlations were stable across the ages 7-13 years. The average correlation of adoptees with biological mothers was 0.17 (95% confidence limits: 0.03, 0.31), with biological fathers, 0.16 (0.00, 0.32), and with biological full siblings at same ages, 0.59 (0.28, 0.90). These correlations were comparable to the correlations within the biological families and comparable to the previously reported correlations for adult adoptees. The correlations with members of the adoptive families were lower--with adoptive mothers, 0.10 (-0.03, 0.23), with adoptive fathers, 0.03 (-0.11, 0.17), and with adoptive siblings, 0.14 (-0.13, 0.41). In conclusion, a genetic influence on body mass index as strong as that in adult life is already expressed by age 7 years. The rearing environment shared by the family has a weak influence during childhood.