Recurrent genomic imbalances at 16p 11.2 are genetic risk factors of variable penetrance for developmental delay and autism.Recently, 16pl 1.2(chr16:29.5 Mb—30.1 Mb) deletion has also been detected in individuals with early-onset severe obesity.The penetrance of 16p11.2 deletion as a genetic risk factor for obesity is unknown.We evaluated the growth and body mass characteristics of 28 individuals with 16p11.2 (chr16:29.5 Mb—30.1 Mb) deletion originally ascertained for their developmental disorders by reviewing their medical records.We found that nine individuals could be classified as obese and six as overweight.These individuals generally had early feeding and growth difficulties,and started to gain excessive weight around 5—6 years of age.Thirteen out of the 18 deletion carriers aged 5 years and older(72%) were overweight or obese,whereas only two of 10 deletion carriers(20%) younger than five were overweight or obese.Males exhibited more severe obesity than females.Thus,the obesity phenotype of 16p11.2 deletion carriers is of juvenile onset,exhibited an age- and gender-dependent penetrance. 16p11.2 deletion appears to predispose individuals to juvenile onset obesity and in this case are similar to the well-described Prader-Willi syndrome(PWS).Early detection of this deletion will provide opportunity to prevent obesity. Recurrent genomic imbalances at 16p 11.2 are genetic risk factors of variable penetrance for developmental delay and autism. Rescently, 16pl 1.2 (chr16: 29.5 Mb-30.1 Mb) deletion has also been detected in individuals with early-onset severe obesity. The penetrance of 16p11 .2 deletion as a genetic risk factor for obesity is unknown. We evaluate the growth and body mass characteristics of 28 individuals with 16p11.2 (chr16: 29.5 Mb-30.1 Mb) Lost originally ascertained for their developmental disorders by reviewing their medical records. We found that nine individuals could be classified as obese and six as overweight. These individuals generally had early feeding and growth difficulties, and started to gain excessive weight around 5-6 years of age. Thirteen out of the 18 missing carriers aged 5 years and older (72%) were overweight or obese, but only two of 10 lost carriers (20%) younger than five were overweight or obese. Sale demonstrated more severe obesity than females.Thus, the ob esity phenotype of 16p11.2 deletion carriers is of juvenile onset, phenotype an age- and gender-dependent penetrance. 16p11.2 deletion appears to predispose individuals to juvenile onset obesity and in this case are similar to the well-described Prader-Willi syndrome (PWS). Early detection of this deletion will provide opportunity to prevent obesity.