Body mass index (BMI) is commonly used to identify obesity. In this study, we determined how accurately BMI could determine body composition and identify obese from non-obese individuals. Fat-free mass and body fat were determined with bioelectrical impedance. Adiposity was calculated as body fat per body mass and as body fat divided by body height (m 2 ). Obesity was defined as a BMI of at least 30 kg/m 2 or an amount of body fat of at least 25% of total body mass for men and at least 30% for women. Obesity as defined by percentage of body fat was always present with a BMI of at least 30 kg/m 2 . However, 30% of men and 46% of women with a BMI below 30 kg/m 2 had obesity levels of body fat. The greatest variability in the prediction of percentage of body fat and body fat divided by height (m 2 ) from regression equations using BMI was at a BMI below 30 kg/m 2 . In conclusion, using impedance-derived body-fat mass as the criterion, people with BMI of at least 30 kg/m 2 are obese. However, significant numbers of people with a BMI below 30 kg/m 2 are also obese and thus misclassified by BMI. Percent of body fat and body fat divided by height (m 2 ) are predictable from BMI, but the accuracy of the prediction is lowest when the BMI is below 30 kg/m 2 . Therefore, measurement of body fat is a more appropriate way to assess obesity in people with a BMI below 30 kg/m 2 .