Attention-deficit/hyperactivity disorder (ADHD) is a genetically as well as environmentally determined disorder with a high rate of psychiatric comorbidity. In this study, non-genetic biological and psychosocial risk factors for ADHD symptom severity and comorbid disorders were assessed in 275 children with ADHD, aged 5-13 years, mean age 9.7 (SD 1.9). Pre-/perinatal biological and lifetime psychosocial risk factors as well as data on parental ADHD were obtained. A different pattern of risk factors emerged for inattentive and hyperactive-impulsive ADHD symptoms. Inattentive symptoms were strongly influenced by psychosocial risk factors, whereas for hyperactive-impulsive symptoms, predominantly biological risk factors emerged. Hyperactive-impulsive symptoms also were a strong risk factor for comorbid oppositional defiant (ODD) and conduct disorder (CD). Smoking during pregnancy was a risk factor for comorbid CD but not ODD and further differential risk factors were observed for ODD and CD. Comorbid anxiety disorder (AnxD) was not related to ADHD symptoms and additional biological and psychosocial risk factors were observed. This study adds to the body of evidence that non-genetic biological and psychosocial risk factors have an impact on ADHD symptom severity and differentially influence comorbid disorders in ADHD. The findings are relevant to the prevention and treatment of ADHD with or without comorbid disorders.