The purpose of this study was to determine the effect of a 10-month treadmill walking program (TWP) on target behaviors (TB: maladaptive behaviors and/or stereotypic mannerisms) in adolescents and young adults with autism. Five youth (3 males, 2 females; 16.6+1.7 yrs) with a primary diagnosis of autism and diagnosed by clinical psychologists as having severe multiple developmental disabilities participated in this study. Participants were involved in a 10-month TWP (10-25 min sessions; speeds 3.3-7.0 km•hr-1, grades 0-2.5%) that was incorporated into their scheduled physical education curriculum 3-5 times per week. Participant’s were monitored 24 hours per day for their three TB. Each behavior was recorded using a tally system. A single-subject quasi-experimental ABÁ research design was used to evaluate each of the participants. In this design, phase A represented a 2-month baseline stage in which behavior was collected but no treadmill was walking was performed. This was followed by a 10-month treadmill walking program (phase B), then another 2-month baseline stage (phase Á). Four of the five participants demonstrated effective results (ie, reduction in TB during phase B). Results of this study indicate that an exercise program (similar to the one in this study), introduced into the daily curriculum of persons with severe autism could positively affect their behavioral goals.