Context: Attention Deficit Hyperactivity Disorder (ADHD) and childhood trauma (CT) are identified in 23% and up to 63% of patients suffering from substance use disorder (SUD), compared with 2.5% and 28%, in the general population, respectively. A growing body of evidence suggests a negative impact of both CT and ADHD on SUD, including an increased severity of the SUD. Yet, the combined influence of CT and ADHD on SUD remains poorly understood. Notably, the type and number of CT could elicit different effects on SUDs in ADHD patients. Objective: To investigate the frequency of combined ADHD and CT in treatment-seeking SUD outpatients and to characterize their combined influence on the severity of SUDs. Methods: outpatients seeking treatment for SUD were consecutively recruited and evaluated for the main SUD diagnosis and severity (number of criteria) using the MINI-S SUD module. ADHD was assessed using the MINI-S ADHD module. CT was assessed using the Childhood Trauma Questionnaire (CTQ), allowing us to consider five trauma types and their cumulative number. In the whole sample and in ADHD vs. non-ADHD patients, we compared the number of DSM5 criteria of the main SUD according to the presence vs. absence of any CT, of each CT type, and as a function of the total number of CT types. We performed Kruskal-Wallis tests, considering statistical significance at p<0.05. Results: From 65 outpatients, 44 (68%) were male and 52 (80%) suffered from severe SUD. Thirty-six (55%) were diagnosed with ADHD, 30 (46%) reported at least one CT type and 19 (30%) reported multiple CT types. The number of CT types were as follows: one, N =11 (37%); two, N =8 (27%), three, N =5 (17%); four, N =5 (17%) and five, N =1 (3%). Emotional neglect was the most reported CT type (N =18, 28%), followed by physical abuse (N =16, 25%), sexual abuse (N =14, 22%), emotional abuse (N =13, 20%) and physical neglect (N =6, 9%). In individuals with ADHD, we observed a clear increase in the number of SUD criteria between patients without CT, with one or two CT types and with more than two CT types (p =0.045). Discussion : The present study underlines the high prevalence of CT and ADHD in patients suffering from SUD. These findings demonstrate how the presence of both ADHD and multiple CT in SUD patients leads to increased SUD severity. This greater severity of TUS could be the result of a maladaptive response adopted in an attempt to better manage ADHD symptomatology and the symptoms engendered by exposure to multiple CT. This study confirms the need for systematic screening for CT and ADHD in the context of SUD management to optimize subsequent treatment. Conflicts of interest : The authors declare no conflict of interest.