The purpose of the present study is to examine the familiality of electroencephalographic (EEG) measures among affected sibling pairs with Attention-Deficit Hyperactivity Disorder (ADHD). EEG was recorded during baseline (eyes open and eyes closed) and cognitive activation conditions on a sample of 58 children with ADHD (27 multiplex families), ages 6–18. EEG power in three frequency bands: theta (4–7 Hz), alpha (8–12 Hz) and beta (12–20 Hz) was tested for sibling correlation, familial co-segregation and association with behavioral task performance on a sustained attention task. Sibling correlation for EEG measures was moderate during baseline conditions and significantly higher for the cognitive activation condition. Familial clustering of frontal and parietal alpha power was evident, but only during the cognitive activation condition. Theta and alpha power correlated significantly with CPT response variability and omission errors, respectively. Cognitive task performance did not exhibit familial clustering in our sample. EEG measures (i.e., alpha power) recorded during cognitive activation is a strongly familial trait in ADHD and may be a putative endophenotype for ADHD. © 2007 Wiley-Liss, Inc.