Anorexia Nervosa (AN) is an eating disorder characterised by distorted cognitions about body weight and shape; but little is known about the phenomenological characteristics of these beliefs. In this study, multidimensional and insight-based measurements were used to compare beliefs about body weight and shape in AN to body image dissatisfaction in the general population, and delusional beliefs in schizophrenia. Twenty participants with clinical and sub-clinical AN, 27 participants with schizophrenia and schizoaffective disorder, and 23 healthy controls completed the Brown Assessment of Beliefs Scale and the Psychotic Symptom Rating Scale in relation to a dominant belief regarding body weight/shape (or body dissatisfaction in healthy controls) or a current delusion. All groups completed the Peters Delusions Inventory to assess the prevalence of a range of delusion-like beliefs. Participants with clinical and subclinical AN experienced significantly higher preoccupation and distress for their belief in comparison to both participants with schizophrenia/schizoaffective disorder rating a delusional belief and the healthy controls rating a belief of body dissatisfaction. Both clinical groups were comparable on ratings of belief conviction and disruption. The data raise questions regarding the current frameworks that are used to describe beliefs in AN.