Family based treatment of anorexia nervosa (AN) requires heavy parental involvement. To be effective, parents must possess a coherent understanding of the illness and believe that it is treatable. Yet we know little of how parents view the illness, nor whether these views are unique to AN. We examined illness perceptions in AN, how they compare to those of individuals with a serious medical illness, and how they relate to adjustment and coping. Twenty-five girls with AN, 24 girls with Type I diabetes, and their mothers completed measures of family functioning and psychological symptoms. Mothers also completed a measure of illness perceptions. Mothers viewed AN as less chronic, understandable, and controllable than mothers of girls with diabetes viewed their daughter’s respective illness. Such negative cognitions were associated with poor family functioning and maternal and adolescent adjustment. These findings have implications for enhancing family based treatments, as well as for modifying public health messages to reduce the pervasive stigma that influences such unhelpful perceptions about the illness.