Attention-deficit/hyperactivity disorder (ADHD) is a childhood neurodevelopmental disorder that can persist into adulthood. The co-occurrence of ADHD and substance use disorders is very frequent and has received considerable attention in recent clinical/scientific investigations. However, few studies have investigated the prevalence of ADHD in heroin addicts. This study aimed to investigate the prevalence of attention-deficit/hyperactivity disorder (ADHD) in a sample of heroin addicts treated with opioid agonists and to report this clinical experience in a public service for addiction. Outpatients over 18 years old and being treated with opioid agonists for heroin addiction were enrolled. Each patient took part in a psychiatric examination and completed an ASRS (Adult ADHD Self-Report Scale) self-assessment. Subjects with positive results were called in for another psychiatric visit, and the Brown ADD scale was used as a second-level test for ADHD; furthermore, the Mini International Neuropsychiatric Interview (MINI) and Hypomania/Mania Checklist (HCL-32) were used for differential diagnoses and to assess comorbidities. In total, 111 patients were enrolled. All were followed up by the psychiatrist, who is also the author of this report and the person who formulated the diagnoses. The prevalence of ADHD in this sample was 18%. Among the 20 patients diagnosed with ADHD, 5 (25%) were female and 15 (75%) were male. The most frequent psychiatric comorbidity was major depression, found in 11 patients (55%), of which 4 presented with hypomania (bipolar disorder). In this sample, making diagnoses was very difficult. Frequently, multiple comorbidities further complicated these cases. In conclusion, the results of this study are consistent with the literature: There seems to be a significant prevalence of ADHD even among heroin addicts, and often, the diagnosis is difficult to make. We also do not know the exact effect of opioid agonist therapy on ADHD symptoms. Hypotheses have been put forward, but studies are needed.