This study evaluated the efficacy of a 9-month treadmill walking (TW) program on exercise capacity and body mass index (BMI) for adolescents with severe autism. Ten youth residing in a residential/school treatment facility were assigned to either a supplemental treadmill walking (TW) or control group. Both groups continued to participate in their regular physical education curriculum. Monthly records were maintained for the following: (a) TW progression in frequency, duration, speed and elevation; (b) caloric expenditure; and (c) BMI. The TW program resulted in significant increases in mean monthly TW frequency, speed, elevation, and calories expended coupled with a reduction in BMI.