Patients with anorexia nervosa frequently believe they are larger than they really are. The precise nature of this bias is not known: is it a false belief related to the patient's aesthetic and emotional attitudes towards her body? Or could it also reflect abnormal processing of the representation of the body in action? We tested this latter hypothesis by using a body-scaled action-anticipation task in which 25 anorexics and 25 control participants had to judge whether or not an aperture was wide enough for them to pass through. The anticipation of body-scaled action was severely disturbed in anorexic patients; they judged that they could not pass through an aperture, even when it was wide enough (i.e. they behave as if their body was larger than in reality). The abnormally high "passability ratio" (the critical aperture size to shoulder width ratio) was also correlated with the duration of illness and the degree of body concern/dissatisfaction. Our results suggest that body size overestimation in anorexia nervosa is not solely due to psycho-affective factors but rather suggest impaired neural processing of body dimensions that might take its source in parietal networks. (Contains 1 figure and 2 tables.)