Introduction: Comorbid attention deficit/hyperactivity disorder (ADHD) is present in 15-25% of all patients seeking treatment for substance use disorders (SUDs). Some studies suggest that comorbid ADHD increases clinical severity related to SUDs, other psychiatric comorbidities, and social impairment, but could not disentangle their respective influences. Objectives: To investigate whether comorbid adult ADHD in treatment-seeking SUD patients is associated with more severe clinical profiles in these domains assessed altogether. Methods: Treatment-seeking SUD patients from 8 countries (N = 1,294: 26% females, mean age 40 years [SD = 11 years]) were assessed for their history of DSM-IV ADHD, SUDs, and other psychiatric conditions and sociodemographic data. SUD patients with and without comorbid ADHD were compared on indicators of severity across 3 domains: addiction (number of SUD criteria and diagnoses), psychopathological complexity (mood disorders, borderline personality disorder, lifetime suicidal thoughts, or behavior), and social status (education level, occupational and marital Icick et al.