The serotonergic system has been implicated in the regulation of impulsive aggressive behavior either toward oneself or others. Imipramine binding sites were measured in the platelets of 23 impulsive aggressive children. Subjects ratings of total behavior, externalizing behavior, hostility, and aggression, as measured by the Child Behavior Checklist, were inversely correlated with the platelet imipramine binding. These findings are consistent with previous studies that suggest that decreased serotonergic activity is associated with impulsive aggressive behavior.