Objective: Attention-deficit hyperactivity disorder (ADHD) often coincides with substance abuse and delinquency. A sample of opioid-maintained inmates was assessed for symptoms of ADHD, substance abuse history, types of offense, psychiatric comorbidities, and psychopharmacological treatment using a standardized battery of instruments. Method: Adult inmates (n = 133, mean age 35.7 years, 21.8% female) in opioid-maintenance therapy (OMT) were administered the Adult ADHD self-report scale, Wender Utah Rating Scale, Mini International Neuropsychiatric Interview, European Addiction Severity Index. Results: Fifty percent screened positive for childhood and 17% for adult ADHD, four (3.1%) received ADHD medication. Inmates with ADHD symptom status were significantly younger at first substance abuse, reported more drug overdoses, longer duration of cocaine and prescribed medication abuse and more in- and outpatient treatments (all p < 0.05). For all inmates in OMT a high rate of psychiatric comorbidities was observed (78.9%). Conclusion: There is a need for assessment of ADHD and other psychiatric comorbidities in OMT prisoners. Evidence-based treatment should be routinely provided.