Objective: Children with ADHD and comorbid disorders display poorer overall functioning compared with children with ADHD alone, though little research has examined the differential impact of externalizing versus internalizing comorbidities. Method: This study examined the impact of internalizing and externalizing comorbidities on the academic and social functioning of 416 children with ADHD. Results: Children with ADHD and a comorbid externalizing disorder (with or without a concomitant internalizing comorbidity) displayed poorer social skills than those with ADHD alone. Children with ADHD and both an externalizing and internalizing comorbidity exhibited greater homework problems than their ADHD peers with fewer than two types of comorbidity. In addition, older children displayed significantly poorer social skills and greater homework problems than younger children. There was no interaction between comorbid status and age for any measure of academic or social functioning. Conclusion: Results underscore the need for early interventions to address social skills and homework difficulties for children with ADHD and comorbid externalizing disorders. (Contains 4 tables.)