OBJECTIVES: Attention-deficit/hyperactivity disorder (ADHD) can have a significant negative impact on health outcomes in adults. This study was designed, in part, to determine the functional outcomes and health care utilization of adult psychiatric outpatients with ADHD in several European countries. METHODS: This was a multinational observational study. All eligible outpatients (excluding patients with any psychotic disorder) from a variety of outpatient settings were invited to participate. ADHD diagnosis was established with the Diagnostic Interview for ADHD in Adults (DIVA) based on the Diagnostic and Statistical Manual of Mental Disorders, 5^thEdition. All patients were further evaluated with the Sheehan Disability Scale (SDS) and the EuroQol‑5 Dimensions (EQ‑5D) questionnaire, which was also used to assess anxiety/depression. RESULTS: Of 5662 patients approached, 2284 (40.3%) enrolled, of whom 1986 patients (87.0%) completed the study. Patients were 17 to 72 (median=42) years of age, and the majority were women (58.8%). Based on the DIVA, 17.4% (95% CI 15.7%-19.0%) of patients were diagnosed with ADHD. Patients with ADHD had moderate to severe overall impairment (mean SDS total score 18.9 [SD=6.6, n=348] versus 11.6 [SD=8.6, n=1659] in patients without ADHD). On the EQ-5D, a majority of patients with ADHD indicated having problems performing their usual activities (66.2% versus 41.2%) and many reported being “extremely anxious or depressed” (24.6% versus 16.0%). However, compared to patients without ADHD (n=1660), patients with ADHD (n=349) were less often prescribed antidepressants (57% versus 71.9%). The proportions of patients who visited a primary care physician, psychiatrist, or psychotherapist during the previous 6 months were similar between the 2 groups. CONCLUSIONS: Adult psychiatric outpatients with ADHD in our sample reported more overall functional impairment and psychiatric comorbidities compared to outpatients without ADHD. The use of medical resources was similar between the 2 groups.