There is increasing evidence both from animal experimentation and from clinical field studies that physical activity can play a central role in the pathogenesis of some eating disorders. However, few studies have addressed the issue of prevalence or whether there are different rates of occurrence across diagnostic categories, and the estimates that do exist are not entirely satisfactory. The present study was designed to conduct a detailed examination of the physical activity history in patients with anorexia nervosa (AN) and bulimia nervosa (BN) both during and prior to the onset of their disorder. A sample of adult patients and a second sample of adolescent AN patients took part in the study. A series of chi-square analyses compared diagnostic groups on a number of variables related to sport/exercise behaviors both premorbidly and comorbidly. Data were obtained by means of a detailed structured interview with each patient. We found that a large proportion of eating disorder patients were exercising excessively during an acute phase of the disorder, overexercising is significantly more frequent among those with AN versus BN, and premorbid activity levels significantly predict excessive exercise comorbidity. These findings underscore the centrality of physical activity in the development and maintenance of some eating disorders. They also have important clinical implications in light of the large proportion of individuals who combine dieting and exercise in an attempt to lose weight, and the increasing recognition of the adverse effects of strenuous physical activity in malnourished individuals.