Attention Deficit / Hyperactivity Disorder (AD/HD) and Conduct Disorder (CD) constitute different diagnostic categories in the DSM-IV. However, several authors propose that CD develops as secondary to ADHD, due to coercive interaction patterns between children with ADHD and their parents that often originate from false parental beliefs regarding child behaviour. The purpose of this study was to compare parental beliefs regarding hyperactivity and conduct problems in the domains of severity, controllability, causality and appropriate parental responses. 317 mothers and 317 fathers of boys and girls aged 4-6, enrolled in kindergartens in Athens, completed the “SDQ” and one version of “The Parental Account of the Causes of Childhood Problems Questionnaire”. Two versions were based on vignettes presenting either a boy or a girl with ADHD and two others included vignettes presenting either a boy or a girl with CD. Conduct problems were perceived as significantly more severe, controllable and impairing than hyperactivity problems. Hyperactivity problems were more likely to be attributed to biological reasons than conduct problems. Finally, strictness was the most likely reaction for conduct problems but not hyperactivity. To conclude, parents usually fail to capture the interrelationship between hyperactivity and conduct problems. Their negative views of the child’s conduct problems may lead to strict and punishing strategies that can cause frequent conflictual encounters. Cognitive-behaviourally-based intervention programs should help parents understand the interrelationship between ADHD symptoms and conduct problems in order for them to adopt more effective responses.