Differences between twins and siblings in behaviour problems were investigated in a non-selected sample of 1938 families with children aged 4-12 years. Families were sent a questionnaire based on DSM-III-R criteria for Attention Deficit Hyperactivity Disorder (ADHD), Oppositional Defiant Disorder (ODD), Conduct Disorder (CD) and Separation Anxiety (SA), which was validated by formal clinical interview. The questionnaire also included measures of speech and reading problems. There were significant differences between twins and siblings for ADHD symptoms, but not for symptoms of ODD, CD or SA. Twins and siblings differed significantly for gestational age, birth weight, speech and reading problems. While there was little evidence for birth weight or gestational age contributing to the difference in ADHD symptoms, there was a strong association between ADHD symptoms and speech and reading problems.