In many sports, athletes with low body weight have a distinct advantage over their opponents; however, this advantage can easily turn into a noticeable disadvantage because low body weight may also be associated with health problems. The present review focuses on the problem of anorexia athletica, with its emphasis on leanness and thinness in female and male sports athletes. Athletes often restrict calories and/or overexercise to achieve or maintain low body and fat masses. There is a growing body of evidence that several metabolic and endocrine disturbances are the result of prolonged energy restriction. However, the long-term outcome of such sport-related disordered eating has not been thoroughly studied. Effective methods of treatment are scarce and similar to treatment of eating disorders. Scientific studies are needed that help establish alternative regulations for sports in which a low body weight is a primary advantage for performance.