Aim: Accurate assessment of nutritional status is a vital aspect of caring for individuals with anorexia nervosa (AN) and body mass index (BMI) is considered an appropriate and easy to use tool. Because of the intense fear of weight gain, some individuals may attempt to mislead the physician. Mid-upper arm circumference (MUAC) is a simple, objective method of assessing nutritional status. The setting is an eating disorders clinic in a tertiary paediatric hospital in Western Australia. The aim of this study is to evaluate how well MUAC correlates with BMI in adolescents with AN.Methods: Prospective observational study to evaluate nutritional status in adolescents with AN.Results: Fifty-five adolescents aged 12–17 years with AN were assessed between January 1, 2004 and January 1, 2006. MUAC was highly correlated with BMI (r = 0.79, P <0.001) and individuals with MUAC ≥20 cm rarely required hospitalisation (negative predictive value 93%).Conclusions: MUAC reflects nutritional status as defined by BMI in adolescents with AN. Lack of consistency between longitudinal measurements of BMI and MUAC should be viewed suspiciously and prompt a more detailed nutritional assessment.