Introduction Attention Deficit Hyperactivity Disorder (ADHD) is found in up to 20% adults with Substance Use Disorder (SUD). ADHD + SUD is associated with a more complex clinical presentation and poorer outcomes than each disorder alone. In the presence of SUD, adult ADHD is particularly difficult to diagnose as both disorders can mimic or hide the symptoms of each other. Our university hospital in Paris recently started an extensive outpatient diagnostic procedure for adult patients with SUD to ascertain or refute ADHD diagnosis and to provide therapeutic guidance. Here, we report the acceptability of the assessment procedure for patients and the preliminary description of the current and lifetime clinical profiles as a function of the final diagnosis “ADHD vs. no ADHD.” Method Adult SUD patients with suspected ADHD were included in the current pilot study after stating they had no objection that their de-identified data were used for research purposes, according to French ethical procedures. Patients were evaluated for ADHD, comorbid mental disorders, cognitive state and dimensional psychological variables. They were assessed by trained psychologists and psychiatrists using standardized tools over a day. ADHD diagnosis was mainly based on the Diagnostisch Interview Voor ADHD for DSM-5 (DIVA-5). Results Out of 18 eligible patients, 17 were included in the cohort (1 excluded) and none was opposed to using their data. Thirteen (76%) participants were diagnosed with ADHD. All patients appointed for the ADHD diagnostic procedure came, respected schedules and finished the evaluation. All patients were impaired on cognitive functioning and were highly comorbid, but ADHD patients seems to suffer even more from those conditions, especially for cannabis and stimulant use disorders. Discussion Preliminary results show high acceptability of the procedure by ADHD-SUD patients. This result could be explained by all the organization adapted to the psychopathology. Patients' baseline motivation to participate also represents an uncontrolled variable that could promote the ability to follow the procedure. Acceptance results of the protocol are promising and represent a starting point to identify the best procedures to design patient-centered pharmacological and non-pharmacological therapies.