Impulsivity in terms of aggression, suicide or poor cognitive control has been associated with low synaptic availability of serotonin (5-HT) in adults and children. However characteristics of the 5-HT transporter have not been studied in children with attention-deficit/hyperactivity disorder (ADHD: combined type) where impulsivity is a core symptom. Here, in 20 children with ADHD, we explore the relationship of the density (Bmax) and affinity (Kd) of the platelet 5-HT transporter measured with [3H]paroxetine to both clinical ratings of impulsivity (Conners’ Parent Questionnaire), and an experimental measure of impulsivity (the ability to withhold a prepotent response in the “stop-signal” paradigm). Decreases of affinity (increased Kd) correlated with a low probability of response inhibition, but not with the clinical ratings of impulsivity. However, ratings of distractibility and impulsivity correlated with the experimental measure of response-inhibition. In contrast, increased transporter affinity (low Kd) correlated modestly with higher ratings of aggressive and externalising behaviour. Bmax was not associated with any behavioural score. We conclude that the synaptic availability of 5-HT is under the control of the 5-HT transporter binding site affinity and that low affinity may be related to cognitive impulsivity (distractibility). Increased affinity of the transporter may also be related to conduct disturbance.