Background: Due to high risk of physical inactivity in children with autism spectrum disorders (ASD) and its adverse consequences, evaluating their level of physical activity (PA) using accelerometry and investigating its relation with age and severity of the disorder are integral.

Materials and Methods: Fifty two boys and sixteen girls aged 7-14 years -from autism schools in Tehran- participated in current cross sectional study. Level of daily PA was evaluated for seven consecutive days using accelerometers. Autism severity was also assessed in the areas of communication, sociability, sensory and cognitive awareness, and health and physical behavior using Autism Treatment Evaluation Checklist (ATEC).

Results: On the whole 91 percent of participants encountered low level of PA. Girls were remarkably less active than boys and were more inactive. Level of PA significantly reduced as they aged. Level of PA was adversely and significantly correlated with autism severity in the area of sensory and cognitive awareness. Moreover, level of vigorous PA was adversely and significantly correlated with total autism severity. Finally, age, sex, and sensory and cognitive awareness were partially predictors of PA.

Conclusion: It appears that children with HFA are physically inactive and considerably engage in more sedentary activities as they age. Age and gender can be considered as physical activity determinants among this population.