We report the case of a 37-year old female patient with anorexia nervosa. For many years, this patient fasted for various periods, from days to weeks, in order to reduce her body weight and to achieve an euphoric effect. In the course of the past four years she developed acute paranoid-hallucinatory psychosis immediately following one week episodes of fasting, requiring psychiatric in-patient treatment three times. Psychotic prodromal symptoms could not be elicited before nor at the beginning of fasting periods, pointing towards a caustic relationship between the fasting and the psychotic decompensation. Based on this case, we discuss the hypothesis that the fasting in anorexia nervosa leads to acute hyperactivity of the dopaminergic system which could explain the development of paranoid-hallucinatory psychosis. This case exemplifies that vulnerable patients with a similar history should strongly be advised to avoid fasting and other dietetic methods. Explanation of the enhanced risk to develop psychosis could be of therapeutic use for treating the anorexia nervosa in such patients.