The aim of this study was to determine the frequency of adult attention deficit hyperactivity disorder (ADHD) comorbidity with lifetime bipolar disorder, and the influence of this comorbidity on various demographic and clinical variables in patients. Patients (n = 159) with a previous diagnosis of bipolar disorder (79 female, 80 male) were included in this study. All patients were interviewed for the presence of current adult and childhood ADHD diagnosis and other axis I psychiatric disorder comorbidities using the structured clinical interview for DSM-IV (SCID) and the Schedule for Affective Disorders and Schizophrenia for School Age Children-Present and Lifetime Version (K-SADS-PL). The subjects also completed a Wender Utah rating scale (WURS-25) and a Current Symptoms Scale for ADHD symptoms. In particular, patients' clinical characteristics, the age of onset of bipolar disorder, and the number of episodes were noted. Twenty-six of the 159 bipolar patients (16.3%) were diagnosed with adult ADHD, while another subgroup of patients (n = 17, 10.7%) received a diagnosis of childhood ADHD but did not fulfill criteria for adult ADHD. Both of these two subgroups (patients with adult ADHD, and patients with only childhood ADHD) had an earlier age of onset of the disease and a higher number of previous total affective or depressive episodes than those without any lifetime ADHD comorbidity. However only bipolar patients with adult ADHD comorbidity had higher lifetime comorbidity rates for axis I psychiatric disorders, such as panic disorder and alcohol abuse/dependence, compared to patients without lifetime ADHD. Bipolar patients with comorbid adult ADHD did not differ from bipolar patients with comorbid childhood ADHD in terms of any demographic or clinical variables except for adult ADHD scale scores. In conclusion, ADHD is a common comorbidity in bipolar patients, and it adversely affects the course of the disease and disrupts the social adjustment of the patients. Regular monitoring of ADHD will help to prevent problems and complications that could arise in the course of the disease, particularly in patients with early onset bipolar disorder.