Obesity impairs performance in most athletic events, but the influence of increased body fat on cardiopulmonary function has not been clearly delineated. An understanding of the fatness-fitness relationship is important in the optimal design of exercise programs for obese subjects. In this study, 27 adolescent females with body fat levels ranging from normal to gross obesity were evaluated to determine the impact of adiposity on physiologic factors during maximal and submaximal treadmill walking. Increased skinfold measures correlated significantly with absolute maximal oxygen uptake throughout the range of body fat levels (r = .72), and oxygen consumption per kilogram of body weight and treadmill endurance time both declined as fatness increased (r = -.49 and -.42, respectively). Obesity did not affect submaximal walking economy. These findings indicate that increased fat levels are associated with increased cardiopulmonary exercise capacity, but that functional fitness declines because of the inert load created by excess body fat. Therefore, therapeutic exercise programs for obese adolescents are best designed to increase caloric expenditure and decrease body fat rather than to improve aerobic fitness.