This paper reviews the literature available in relation to biological
basis of attention deficit-hyperactivity disorder (ADHD). Twin, adopti
on, family-genetic and segregation analysis studies have emphasized th
e existence of an important genetic contribution to the etiology of AD
HD. Overall, data are concordant with the notion of a poligenetic inhe
ritance. On the other hand, from a neurochemical point of view, severa
l neurotransmitters have been involved in the genesis of this disorder
. The fact that stimulant drugs act as dopaminergic and noradrenergic
agonists has lead to the ''catecholamines hypothesis''. However, no co
mprehensive model has been proposed which successfully describes the u
nderlying pathophysiology of ADHD and the mechanisms by which psychost
imulants ameliorate the symptomatology. Likewise, we examine several n
euroendocrinological investigations which have demonstrated a dysfunct
ion of thyroid system in the ADHD as well as the main studies using st
ructural and functional neuroimaging techniques which have showed a cl
ear fronto-striatal-cortical dysfunction in these patients. Finally, w
e describe some recent neuroimmunological studies which indicate that
C4B levels may be an important marker for ADHD.