The goal was to analyze the sex differences in symptoms, comorbidity and functional impairment in outpatient children with Oppositional Defiant Disorder (ODD). A sample of 343 children, aged 8 to 17 years and diagnosed with ODD, were assessed with a semi-structured diagnostic interview and dimensional measures of psychopathology and functional impairment. Boys with ODD more frequently displayed the symptoms "deliberately annoys" and "blames others", presented comorbid ADHD, and had greater functional impairment in school and community contexts; girls presented higher comorbidity with internalizing symptomatology (anxiety, depression and somatic complaints). Given that some clinical differences are apparent in ODD between boys and girls, it is necessary to consider the sex of the patient in order to identify and treat this disorder efficiently and effectively in boys and in girls.