Anorexia nervosa (AN) is a serious mental illness, characterized in part by intense and irrational beliefs about shape and weight, including fear of gaining weight. Although these beliefs are considered to be a diagnostic criterion for the illness, they have not been systematically characterized. This study used the Brown Assessment of Beliefs Scale (BABS) to identify the dominant belief that interfered with eating in a sample of underweight patients with AN (N=25). The degree of insight was assessed quantitatively. The majority of participants (68%) spontaneously reported a dominant belief consistent with fear of gaining weight or becoming fat. Twenty percent of patients were categorized as delusional. The total score on the BABS was significantly correlated with the drive-for-thinness subscale of the Eating Disorders Inventory (EDI) (r=0.41, p=0.04), but did not correlate with overall measures of AN severity (body mass index [BMI], duration of illness, lowest BMI, other subscales of the EDI, or total EDI score). These findings highlight the centrality of fear of fat in AN and suggest the possibility that there is a subgroup of patients whose concerns about their weight reaches delusional proportions. This subpopulation of patients warrants further study, since patients with more delusional beliefs may have a form of AN that is more refractory to treatment.