Anorexia nervosa is the most deadly mental disorder. Early detection is crucial. Most patients engage in compulsive exercise, often in an early stage. Compulsive exercise has a deleterious impact on the course of the disease. It can manifest in extreme forms with rigid rules and an inevitable urge. Three different subtypes exist: intensive sports, increased movement in everyday life, and motor restlessness. Besides the underlining motives, neurobiological mechanisms play a role. Moreover, cold ambient temperature increases compulsive exercise, while many patients intentionally expose themselves to cold. Compulsive exercise disappears only partly after weight recovery. Therefore, it needs specific treatment. Psycho-education is indispensable: to learn the difference between healthy and compulsive exercise. Besides decreasing excessive exercising, supervised exercise training is helpful. In short, think of AN in case of compulsive exercise and think of compulsive exercise of case of AN. Pose the right questions to get an impression of the severity.