Promotion of physical fitness and an active lifestyle is critical during the formative adolescent years. This study investigated the effect of a school-based, alternative physical education program of walking training on the aerobic fitness of sedentary, predominantly obese teenagers. The subjects trained three days a week for 11 weeks at an average pace of 3.9 mph and heart rate of 151 beats per minute (79.6% of maximum). Physiologic responses during maximal treadmill testing before and after walking training were compared to those during a 3-month control period preceding the program. Weight-relative maximal oxygen uptake improved 9.9 percent (p less than .05), while treadmill endurance time increased by two minutes (23 percent, p less than .05). These findings indicate that small but significant improvements in aerobic function can be expected from walking training in sedentary adolescents.