Adult family members resemble each other in body mass index (weight/height2). Our aim was to assess the extent to which the genetic relationship can account for the resemblance in natural families. We estimated the correlations in body mass index between adult adoptees, their adoptive parents, and their biological parents and siblings and compared the correlations to those between the biological relatives. Height, current weight, and greatest ever weight were obtained for 3651 adoptees. Groups representing thin, medium weight, overweight and obese subjects were selected by current body mass index (n = 540) and similarly selected by maximum body mass index (n = 524). The heights and weights of their biological and adoptive family members were assessed. The correlation in current body mass index between the adoptees and biological mothers, fathers, and full siblings were 0.15, 0.11, and 0.23, respectively (all P less than 0.001). These correlations were compatible with those found among the biological relatives living together. The correlations exhibited no consistent gender-related pattern. There was no correlation in body mass index between the adoptees and their adoptive parents. Similar results were obtained for maximum body mass index. Moreover, the correlations between the adoptees and their biological relatives are very similar to those found in large population studies of natural families. This study suggests that the genetic relationship fully accounts for the familial resemblance in body mass index among adults.