Bipolar II disorder and ADHD share several clinical characteristics. Identifying patients with either Bipolar II or ADHD is therefore not an easy task. Little is known about the co-occurrence of both disorders and its treatment.

In a large outpatient clinic for Mood Disorders all patients with a bipolar II disorder were asked to fill in the ADHD rating scale, a screening instrument for adult ADHD. Patients who were above threshold were asked to participate in further diagnostics. This included a semi-structured interview for adult ADHD and an interview with an important other. Outcome was rated by two independent experts in adult ADHD.

The total sample consisted of 62 bipolar II patients. Forty-two participated in the first screening. The ratings of 22 patients were not above threshold. Of the 20 patients with a positive score, 6 refused further participation. Of the 14 remaining, 11 satisfied full ADHD criteria in childhood as well as adulthood.

Patients with co-occurring ADHD were significantly more often female (82%) and had more relationships in the past. All other demographic variables were not significantly different. None of the treating physicians had prior to the study been aware of the diagnosis of ADHD in the bipolar II patients.

In adults with bipolar II disorder, ADHD is a common co-occurring disorder. Almost 18% of patients with bipolar II disorder also applied for a lifetime diagnosis of ADHD. Especially female patients with bipolar II disorder had relatively often co-occurring ADHD. Since this study was carried out in a specialized centre for mood disorders, further confirmation of this high prevalence rate should be object of further study.