Dysregulation of serotonergic function has been found to be associated with aggression in animals, human adults and adolescents. However, studies with children have shown conflicting results. The objective of this study was to investigate whether the kinetic characteristics (Vmax and Km) of 5-HT uptake in platelets are different in children with the diagnosis of conduct disorder according to ICD-10 and healthy age-matched controls. In addition to the standardized assessment of general psychopathology, methods assessing narrowband aggressive symptoms (Child Behavior Checklist) and emotional reactivity to an experimentally induced provocation (Taylor's competitive reaction time task) were used in both groups. We found a trend for a lower mean Vmax of platelet 5-HT uptake in 14 conduct-disordered boys compared with healthy controls (n=15). If, however, 2 patients with a low degree of aggression and emotional reactivity were excluded, the difference became significant (mean=4.27, SD=3.49 in patients and mean=8.45, SD=4.63 in controls). A significant negative correlation was found between parent-rated aggression scores and Vmax (r=-0.41, p < 0.05, n=29). These data suggest that dysfunction of 5-HT transport mechanisms might be associated with specific behavioral symptoms in conduct-disordered children.