Obesity in adults is associated with an increased mortality rate from various diseases. Childhood overweight or obesity may persist into adulthood, and for this reason it is important to identify such children at risk. The data were taken from a larger longitudinal growth study of 3650 full-term and healthy Swedish babies followed from birth to 18 y of age. Body mass index (BMI) was used to estimate (during the pediatric years) the risk of obesity at 18 y of age. A probability chart for becoming overweight (>25 kg/m2) by 18 y of age was constructed, For example, in girls, a BMI of 16 kg/m2 at 4 y of age is associated with a 20% risk of attaining a BMI value over 25 kg/m2 at 18 y, and in boys at 4 y of age, a BMI of 19 kg/m2 leads to a 60% risk that they will have a BMI value over 25 kg/m2 at 18 y. The probability risk charts for adult overweight developed here is the first one presented. They provide an easy and novel instrument to use to identify those children who are at high risk of becoming obese adults, so that they may have clinical intervention at younger ages.