This study aimed to characterize the nature and impact of disruptive mood dysregulation disorder (DMDD) in children with attention-deficit/hyperactivity disorder (ADHD) including its co-occurrence with other comorbidities and its independent influence on daily functioning. Children with ADHD (6-8 years) were recruited through 43 Melbourne schools, using a 2-stage screening (parent and teacher Conners 3 ADHD index) and case-confirmation (Diagnostic Interview Schedule for Children, Version IV; [DISC-IV]) procedure. Proxy DMDD diagnosis was confirmed via items from the oppositional defiant disorder (ODD) and major depressive disorder modules of the DISC-IV. Outcome domains included comorbid mental health disorders, academic functioning, social functioning, child and family quality of life, parent mental health, and parenting behaviors. Unadjusted and adjusted linear and logistic regression were used to compare children with comorbid ADHD and DMDD and children with ADHD without DMDD. Thirty-nine out of 179 children (21.8 %) with ADHD had comorbid DMDD. Children with ADHD and DMDD had a high prevalence of ODD (89.7 %) and any anxiety disorder (41.0 %). Children with ADHD and DMDD had poorer self-control and elevated bullying behaviors than children with ADHD without DMDD. Children with ADHD and DMDD were similar to children with ADHD in the other domains measured when taking into account other comorbidities including ODD. One in five children with ADHD in their second year of formal schooling met criteria for DMDD. There was a very high diagnostic overlap with ODD; however, the use of a proxy DMDD diagnosis containing items from the ODD module of the DISC-IV may have artificially inflated the comorbidity rates. DMDD added to the burden of ADHD particularly in the area of social functioning.