Hyperactivity is frequently observed in eating disorders, and several biopsychological mechanisms have been proposed to explain its pathogenetic role. In view of the lack of a reliable method to study hyperactive behavior, we did an experiment with experience sampling methodology (ESM). During 1 week, an anorexia nervosa (AN) patient was asked at nine random times a day to report her momentary tendency to be physically active, her emotions and several other variables including calorie expenditure, drive for thinness, attractiveness, obsessions, compulsions, and attitudes towards hyperactivity. Results indicate that the patient's tendency to be hyperactive was (a) positively related to her weight preoccupation and her negative emotions, and (b) negatively related to her positive emotions and the absence of depression. In this patient, obsessions and compulsions were not related to hyperactivity. The usefulness of ESM for studying the role of hyperactivity in AN is discussed.