This study investigated the prevalence of obesity in 1671 Caucasian 11-14 year olds using body mass index (BMI) and fat percentage (F%). Age and sex-related criteria for both BMI and F% were applied to characterise the individuals as normal, overweight/overfat or obese. Using BMI, 5.6% males and 6.1% females were identified as obese. F% gave higher values for obesity, 11.9% males and 15.3% females. The prevalence of obesity appears to be in-line with other surveys however; the outcome does appear to depend on both the measure employed (BMI/F%) and the categories used (overweight/obese). Questions are raised by this survey as to whether BMI is underestimating the number of individuals who are obese, given that obesity is a factor of adiposity, and whether the two measures are identifying the same individuals as obese; 23% were categorised differently between the two measures. These issues have implications for both survey and clinical work.