ObjectivesZZThis study was designed to investigate the clinical availability of quantitative electroencephalography (QEEG) as an auxiliary diagnostic tool in attention-deficit hyperactivity disorder (ADHD). MethodsZZA total 95 participants completed examinations, which included Korean ADHD Rating Scale (K-ARS), Diagnostic Interview Schedule for Children Version IV-Korean Version (DISC-IV), and QEEG. From the result of the DISC-IV, we divided them into three groups, ADHD, ADHD NOS (not otherwise specified), and Normal control. The QEEG was analyzed by the ranges of Hz : delta (1-4 Hz), theta (4-8 Hz), alpha (8-12 Hz), beta (12-25 Hz), and high beta (25-30 Hz). ResultsZZADHD NOS group showed significantly decreased delta activity (NOS<normal, p=0.003) and increased theta activity (NOS>normal, p=0.044) compared with normal control. ADHD group showed increased high beta activity (ADHD>NOS, p=0.043) compared with NOS group. The z-scores of relative power of theta were negatively correlated with the K-ARS at O1, O2 electrodes of ADHD group. On the other hand, the z-scores of relative power of high beta were negatively correlated with the K-ARS at F7, F8 electrodes of ADHD NOS group. ConclusionZZWe confirmed QEEG abnormalities in ADHD patients, especially the difference between ADHD NOS and Normal control. Therefore, we expect to use QEEG as the valuable tool to diagnose ADHD accurately.