The objective of this study was to evaluate the accuracy of body mass index (BMI) in the diagnosis of obesity. The relationship of this weight-for-height index to body composition was determined using dual-energy x-ray absorptiometry (DEXA) in 226 Caucasian subjects. BMI-obesity was defined as a BMI greater than 27.8 kg/m2 in males and greater than 27.3 kg/m2 in females. BMI-obesity was weakly sensitive (males: 12.5%, females 13.6%) compared with diagnosis by DEXA, defined as percent body fat mass (%BF) greater than 20% in males and greater than 25% in females. Conversely, the specificity of BMI-obesity was high (males and females: 100%). When analyzing subgroups of subjects according to weight, sensitivity was higher among heavier subjects than among lighter ones. In both sexes and in all subgroups, the specificity was 100%. The clinical implication of this spectrum bias is that, in men or women weighing less than 80 kg, measures of obesity other than BMI, such as bioelectrical impedance, should be preferred.