Aims: To examine the role of attention deficit/hyperactivity disorder (ADHD) and impulsive personality disorders in nicotine addiction severity among treatment-seeking substance use disorder (SUD) patients. Methods: In a cross-sectional study, we examined data from the second International ADHD in Substance Use Disorders Prevalence Study (IASP-2) on 402 adults in SUD treatment from Puerto Rico, Hungary, and Australia using diagnostic interviews for ADHD, antisocial (ASP) and borderline (BPD) personality disorders, and the self-report Fagerström Test of Nicotine Dependence (FTND). We compared SUD patients with and without ADHD on nicotine addiction severity. We tested direct and indirect pathways from ADHD to nicotine addiction and mediation Sánchez-García et al.