The current study tests electroencephalogram (EEG) measures as a potential endophenotype for attention deficit/hyperactivity disorder (ADHD) by examining sibling and parent-offspring similarity, familial clustering with the disorder, and association with the dopamine receptor D4 (DRD4) candidate gene.The sample consists of 531 participants (191 parents and 340 children) from 132 multiplex families with ADHD who participated in a larger genetics study. All members of the families underwent extensive assessment including semi-structured diagnostic interviews and EEG recording.Strong sibling similarity and parent-offspring correlations in EEG power emerged, suggesting high trait heritability. Increased theta power was observed among children with ADHD when compared with unaffected children, and there were no differences according to ADHD subtype. Within the parent sample, ADHD diagnostic status and ADHD subtype group differences emerged in the theta, alpha, and beta frequency bands. DRD4 effects for both parents and children were apparent in the beta frequency band and for children only in the theta frequency band.This study suggests that EEG measures are a promising avenue of study in the search for putative endophenotypes for ADHD, and that variability at the DRD4 gene may contribute to this endophenotype.