Americans are becoming overweight at an alarming rate. In 1960, about 31% of Americans were overweight and 13% were obese. By 1999, 35% of Americans were overweight and 26% were obese. Body mass index (BMI) is a way to measure whether a person is of appropriate weight. BMI is calculated by dividing a person's weight in kilograms by the square of his or her height in meters (BMI = weight in kilograms/height in meters 2). An appropriate BMI is between 18.5 kg/m2 and 25 kg/m2. People with BMIs less than 18.5 kg/m2 are underweight, people with BMIs between 25 and 30 kg/m2 are overweight, and people with BMIs 30 kg/m2 or greater are obese. Obesity is a serious public health problem because obese people are more likely than normal-weight people to suffer from chronic health problems, such as high blood pressure or diabetes, and to die at younger ages. Knowing at what age people are likely to become obese and which groups are at high risk for obesity will be helpful in developing ways to prevent this serious health problem.