Gilles de la Tourette syndrome and conduct disorder (CD) are both heterogeneous childhood onset conditions, and although patients with CD have been described in Gilles de la Tourette syndrome cohorts, little is known about the etiology of CD in Gilles de la Tourette syndrome or of the interrelationships. A cohort of 578 consecutive patients with Gilles de la Tourette syndrome was assessed using standard assessment protocols. A total of 13.5% of participants had only Gilles de la Tourette syndrome, whereas the rest had associated comorbidities and psychopathology. CD occurred in 14.5% of Gilles de la Tourette syndrome probands. These findings suggest that CD is not an integral part of Gilles de la Tourette syndrome but rather that CD in the context of Gilles de la Tourette syndrome is related to the presence of attention deficit hyperactivity disorder, as well as, and importantly, a family history of aggressive and violent behavior and forensic encounters.