Previous studies have shown high rates of comorbid disorders in children and adolescents with autism spectrum disorder, but failed to compare them with general population and few of them have identified predictors of comorbidity. This study compared the rates of psychiatric disorders in 50 children and adolescents with autism spectrum disorder, 24 of their siblings, 32 controls from general population and 22 of their siblings. Children and adolescent with autism spectrum disorder and their siblings had higher rates of attention deficit and hyperactivity disorder compared to controls. Lower socioeconomic status and intelligence quotient were the main risk factors. The contribution of socioeconomic status and intelligence quotient to increase the risk of developing comorbidity in autism spectrum disorder and psychopathology in their siblings deserves further study.