This systematic review and meta-analysis investigates the ability of simple measures of childhood obesity, primarily body mass index (BMI), to predict future obesity in adolescence and adulthood. The study included 15 prospective cohort studies with a total of 200,777 participants. The results show that obese children and adolescents are approximately five times more likely to be obese in adulthood compared to those who are not obese. Specifically, around 55% of obese children become obese in adolescence, 80% of obese adolescents remain obese in adulthood, and 70% remain obese over the age of 30. However, 70% of obese adults were not obese in childhood or adolescence, indicating that targeting obesity reduction solely at obese or overweight children may not significantly reduce the overall burden of adult obesity. The review also found that BMI is less effective at predicting adult obesity, as 70% of obese adults were not obese in childhood or adolescence. The study highlights the importance of early intervention to reduce obesity in adolescents, but cautions that targeting interventions at younger children may not have a substantial impact on reducing adult obesity.This systematic review and meta-analysis investigates the ability of simple measures of childhood obesity, primarily body mass index (BMI), to predict future obesity in adolescence and adulthood. The study included 15 prospective cohort studies with a total of 200,777 participants. The results show that obese children and adolescents are approximately five times more likely to be obese in adulthood compared to those who are not obese. Specifically, around 55% of obese children become obese in adolescence, 80% of obese adolescents remain obese in adulthood, and 70% remain obese over the age of 30. However, 70% of obese adults were not obese in childhood or adolescence, indicating that targeting obesity reduction solely at obese or overweight children may not significantly reduce the overall burden of adult obesity. The review also found that BMI is less effective at predicting adult obesity, as 70% of obese adults were not obese in childhood or adolescence. The study highlights the importance of early intervention to reduce obesity in adolescents, but cautions that targeting interventions at younger children may not have a substantial impact on reducing adult obesity.