Abstract Attention-deficit hyperactivity disorder (ADHD), a syndrome that typically first appears in early childhood, can occur in individuals of all ages. Prospective studies have demonstrated that at least half of children diagnosed as having ADHD continue to suffer the symptoms of this disorder in their adult life with significant impacts on their social status, achievement level and sense of well-being. The purpose of this preliminary study was to determine the rate of ADHD in patients with bipolar disorder (BD) and to examine the effects of comorbid ADHD on several clinical and sociodemographic variables of bipolar patients. Forty-four BD-I patients followed up in psychiatric outpatient clinics in two university hospitals, were assessed for the presence of adult ADHD according to DSM-IV. All patients also completed the Wender Utah Rating Scale for objective evaluation of ADHD. Of 44 patients with BD-I, only seven (15.9%) fulfilled criteria for a diagnosis of adult ADHD. Bipolar disorder-I patients with comorbid ADHD were more likely to be female, and have more affective episodes (especially depressive episodes) than bipolar patients without comorbid ADHD. Age at onset of affective illness was not significantly different between the two groups. In line with results of several previous reports, the present study also showed higher prevalence of ADHD in patients with BD-I than in normal population. A higher number of affective episode in patients with comorbid ADHD may suggest a more severe clinical course of BD in these patients. A larger group of samples is required to clarify the exact association and interaction between these two clinical entities.