The primary weight criterion for a diagnosis of anorexia nervosa is a weight less than 85% of what is considered normal for that person’s age and height (DSM-IV and ICD-10). According to DSM-IV, a body mass index less than or equal to 17.5 kg/m2, which originated from the ICD-10 diagnostic criteria for research, is an alternative and somewhat stricter guideline. However, this alternative criterion is not adjusted for age and sex. Because anorexia nervosa typically begins in late childhood, adolescence, or early adulthood, it is crucial to consider age in making a diagnosis because the relation of weight to height changes substantially during this age span. This is highlighted by the fact that with increasing age, the proportion of individuals with a body mass index less than or equal to 17.5 drops dramatically from 57% at 10 years to below 1% at age 35 in the German female population (similar percentages apply to the U.S. population) (1).