Recent studies suggest that Attention Deficit Hyperactivity Disorder (ADHD) is a common comorbid condition in childhood epilepsy, but little is known regarding the nature, frequency and timing of associated neurobehavioural/cognitive complications or the underlying aetiology of ADHD in epilepsy. This investigation examined: (i) the prevalence of ADHD and its subtypes; (ii) the association of ADHD with abnormalities in academic, neuropsychological, behavioural and psychiatric status and (iii) the aetiology of ADHD in paediatric epilepsy. Seventy-five children (age 8-18) with new/recent onset idiopathic epilepsy and 62 healthy controls underwent structured interview (K-SADS) to identify the presence and type of DSM-IV defined ADHD, neuropsychological assessment, quantitative MR volumetrics, characterization of parent observed executive function, review of academic/educational progress and assessment of risk factors during gestation and delivery. The results indicate that ADHD is significantly more prevalent in new onset epilepsy than healthy controls (31% versus 6%), characterized predominantly by the inattentive variant, with onset antedating the diagnosis of epilepsy in the majority of children. ADHD in childhood epilepsy is associated with significantly increased rates of school based remedial services for academic underachievement, neuropsychological consequences with prominent differences in executive function, and parent-reported dysexecutive behaviours. ADHD in paediatric epilepsy is neither associated with demographic or clinical epilepsy characteristics nor potential risk factors during gestation and birth. Quantitative MRI demonstrates that ADHD in epilepsy is associated with significantly increased gray matter in distributed regions of the frontal lobe and significantly smaller brainstem volume. Overall, ADHD is a prevalent comorbidity of new onset idiopathic epilepsy associated with a diversity of salient educational, cognitive, behavioural and social complications that antedate epilepsy onset in a significant proportion of cases, and appear related to neurodevelopmental abnormalities in brain structure.