Objective: To characterize the psychiatric, psychosocial and cognitive functioning of adolescent ADHD females in comparison with female controls and ADHD males. Female controls were also compared to male controls to verify gender differences in a nonclinical sample.Methods: 107 adolescents from Southern Ontario aged 13 to 16 were included: 24 ADHD females, 35 ADHD males, 28 control females and 20 control males. All were assessed using semi-structured interviews, questionnaires and tests of achievement and intellectual functioning.Results: After controlling for parental education and estimated FSIQ, ADHD females were more impaired than control females in: depression, anxiety, distress, teacher relationships, stress, attributional styles, locus of control, and on all cognitive and achievement measures. ADHD females were more impaired than ADHD males in self-reported anxiety, distress, depression, locus of control, and vocabulary scores. These group differences were confirmed by higher ratings by parents and teachers in symptoms of psychopathology. ADHD males were more impaired in processing speed. Some gender differences (locus of control and vocabulary scores) were eliminated when controlling for ADHD severity. The absence of any differences between male and female controls indicates gender differences were specific to the clinical groups.Conclusion: ADHD females are at high risk for more psychological impairment than both ADHD males and control females. The identified psychosocial problems point to areas for intervention.