Attention-deficit-hyperactivity disorder (ADHD) is a common neurodevelopmental disorder with a high degree of associated behavioural problems. In order to study characteristics of ADHD with and without oppositional defiant disorder (ODD) in a representative group of young children with clinical impairment in Sweden, 131 children (101 males, 30 females) with ADHD (mean age 5 years, SD 1 year 5 months; range 3 to 7 years) were clinically examined, and their parents interviewed. Independent questionnaire data (Child Behavior Checklist, ADHD Rating Scale-IV, ODD Rating Scale-IV, Conners Hyperactivity Index) were collected. For comparison 131 children without ADHD were matched for sex, age, marital status, and socioeconomic status (115 males, 16 females; mean age 4 years 10 months, SD 1 year 5 months; range 3 to 7 years). Sixty percent of those with ADHD met full DSM-IV criteria for ODD. Only 10 of the 131 children with ADHD had no symptoms of ODD at all. The rate of children meeting full diagnostic criteria for ODD was similar across all age cohorts. Males were overrepresented in ODD, as were children of divorced parents and of mothers with low socioeconomic status. ADHD combined subtype was a stable independent factor influencing the diagnosis of ODD, regardless of psychosocial factors. Those with ADHD with ODD consistently showed higher rates of ADHD symptoms than did those with ADHD without ODD. The prevalent comorbidity of ADHD with ODD indicates that all children presenting with ADHD or ODD symptoms need to be assessed with a view to exploring both types of problem behaviours. The link between ODD and some psychosocial variables indicates the need to address these, possibly by measures such as parent training and network support.