This study compares "obligatory" runners (runners who continue to run despite clear physical injury or contraindications) to nonobligatory runners. Both groups scored within the normal range on most psychological test indices. The two groups had more similarities than differences. The obligatory runners did present more significant elevations of the Minnesota Multiphasic Personality Inventory scales than did the nonobligatory runners. Based on their responses to the semistructured interview, the obligatory runners were significantly more concerned and rigid about weight control than the nonobligatory runners and the obligatory runners were more likely to prefer being alone. Obligatory runners were more preoccupied with their bodies and reported more positive changes in self-concept and a greater sense of control over their lives since they had begun to run. Female runners reported more satisfaction with and more positive effects from running than did male runners. Strenuous exertion is known to increase prolactin in male and female runners. Clinical research studies suggest that an increase in prolactin is associated with an obsessive preoccupation with diet and/or exercise.