Reliable measures of body composition are essential to develop effective policies to tackle obesity. The lack of an acceptable gold-standard for measuring fatness has made it difficult to evaluate alternative measures of obesity. We use latent class analysis to characterise existing diagnostics. Using data on US adults we show that measures based on body mass index and bioelectrical impedance analysis misclassify large numbers of individuals. For example, 45% of obese White women are misclassified as non-obese using body mass index, while over 50% of non-obese White women are misclassified as being obese using bioelectrical impedance analysis. In contrast the misclassification rates are low when waist circumference is used to measure obesity. These results have important implications for our understanding of differences in obesity rates across time and groups, as well as posing challenges for the econometric analysis of obesity.