The aim of this study was to evaluate the psychiatric comorbidities and different areas of life functioning in substance abusers with attention deficit hyperactivity disorder symptoms. A cross-sectional, multi-center study involving 285 adult substance abusers from outpatient and inpatient clinics was performed. The Adult ADHD Self-Report Scale, the sixth version of the Addiction Severity Index, and the Mini International Neuropsychiatry Interview were used for data collection. Individuals with comorbid attention deficit hyperactivity disorder and substance use disorders showed increased addiction severity when compared with individuals without attention deficit hyperactivity disorder (53.3 ± 7.3 vs. 48.4 ± 8.4, respectively). Our results suggest that comorbid attention deficit hyperactivity disorder and substance use disorders is associated with a more severe course of substance use and with social and psychiatric impairment.