Attention deficit/hyperactivity disorder (ADHD) often presents as an impairing lifelong condition in adults, yet it is currently under-diagnosed and under-treated in many European countries.

To establish the characteristics of the European (EU) adult ADHD patient relative to adult patients outside the EU (OEU).

To compare the baseline characteristics of patients with ADHD in regions where adult ADHD is relatively well established (e.g., USA), with EU adult ADHD patients.

Baseline data was used from the open-label acute treatment period of a multicenter, randomized, withdrawal trial of atomoxetine in adult patients with ADHD (N = 2017; EU, n = 1217; OEU, n = 800). All enrolled patients were included in the baseline analyses.

The demographics for patients in the EU region and regions OEU were comparable. Patients in the EU region had a somewhat lower percentage of prior exposure to psychostimulants compared to the region OEU (32.7% versus 38.9%, p = .005). Conners’ Adult ADHD Rating Scale-Investigator Rated: Screening Version with adult ADHD prompts (18 item total, inattentive and hyperactive/impulsive subscales, and index) were comparable. The adult ADHD Quality of Life life outlook and life productivity domain scores were different between groups (p ≤ .0004). The EuroQol-5 Dimension UK and US population-based Index score, and health state score were comparable between groups.

There were some subtle differences between study groups; however, overall, the adult ADHD patients were not substantially different between the EU region and regions OEU, suggesting that baseline features of ADHD in adult EU patients manifest comparable to those in patients OEU.