BACKGROUND: To examine the comorbidity of ADHD in association with family environment and the severity of ADHD. METHOD: A screening for ADHD symptoms was conducted among adolescents in the Northern Finland 1986 Birth Cohort (N = 6622). A sample of those adolescents (n = 457), aged 16-18 years, with and without ADHD symptoms was assessed with a diagnostic interview (Kiddie-SADS-PL) and ADHD and comorbid disorders were studied in association with the family characteristics and the number of ADHD symptoms. RESULTS: Adolescents with ADHD had more commonly conduct disorder (P