Background. An obstacle to the successful classification of attention deficit hyperactivity disorder (ADHD) is the frequently reported co-morbidity between ADHD and conduct disorder (CD). Prior work suggested that from a familial perspective, ADHD children with CD may be aetiologically distinct from those without CD.

Methods. Using family study methodology and three longitudinal assessments over 4 years, we tested hypotheses about patterns of familial association between ADHD, CD, oppositional defiant disorder (ODD) and adult antisocial personality disorder (ASPD).

Results. At the 4-year follow-up, there were 34 childrenwith lifetime diagnoses of ADHD + CD, 59 with ADHD + ODD and 33 withADHD only. These were compared with 92 non-ADHD, non-CD, non-ODDcontrol probands. Familial risk analysis revealed the following: (1)relatives of each ADHD proband subgroup were at significantly greaterrisk for ADHD and ODD than relatives of normal controls; (2) ratesof CD and ASPD were elevated among relatives of ADHD + CD probandsonly; (3) the co-aggregation of ADHD and the antisocial disorders couldnot be accounted for by marriages between ADHD and antisocial spouses;and (4) both ADHD and antisocial disorders occurred in the samerelatives more often than expected by chance alone.

Conclusions. These findings suggest that ADHD with and without antisocial disorders may be aetiologically distinct disorders and provide evidence for the nosologic validity of ICD-10 hyperkinetic conduct disorder.