The aim of the present proposal is to prove that adult attention deficit hyperactivity disorder (ADHD) patients show lower serotonin transporter (5-HTT) binding using positron emission tomography (PET) and the selective radioligand [11C]DASB. Specifically, the 5-HTT binding will be quantified in 20 adult medication-free ADHD patients (50% females) and in 20 age- and sexmatched healthy controls. Investigating untreated adult ADHD patients without any psychiatric comorbidities will provide the opportunity to estimate the change of serotonin transporter binding in adult ADHD patients compared to a group of healthy controls. Several lines of evidence support the hypothesis that serotonergic neurotransmission may, in addition to dopamine, play an important role in the aetiology of ADHD. So far, no PET study investigating serotonergic neurotransmission in adult ADHD patients has been conducted, although alterations in the serotonin system may be substantially involved in the susceptibility and subtype characterization of ADHD.