This study aimed to assess the prevalence of bipolar disorder (BPD) in children and adolescents with attention deficit hyperactivity disorder (ADHD), and to compare the clinical characteristics of a group with ADHD with a group with co-morbidity of ADHD and BPD. The study includes 121 individuals, aged 6-16 years, with a diagnosis of ADHD. Co-morbidity of BPD was evaluated using the Schedule for Affective Disorders and Schizophrenia for School-age Children-Present and Lifetime version (K-SADS-PL) and the Parent-Young Mania Rating Scale (P-YMRS). The Child Behavior Checklist (CBCL) was used to assess psychopathology in two groups. Ten children (8.3%) in the ADHD sample received the additional diagnosis of BPD. The ADHD + BPD group had significantly higher scores than the ADHD group on withdrawn, anxiety/depression, social problems, thought problems, attention problems, aggression, externalization, total score items of CBCL, and on the P-YMRS. It could be concluded that BPD is not a rare co-morbid condition in children with diagnosis of ADHD and subjects with this co-morbidity show more severe psychopathology than subjects with pure ADHD. Differential diagnosis of BPD disorder in subjects with ADHD seems crucial in establishing an effective treatment program, and therefore improving mental health outcomes.