Objectives: This study aimed to: (1) identify clusters based on the diagnostic status of mental disorders among individuals with problematic substance use treated in an addiction rehabilitation center (ARC), and (2) characterize these clusters according to socio-demographic variables, neighborhood characteristics, and health service utilization. Methods: Data were compiled for 4526 individuals with problematic substance use who received services from an ARC in 2004. The data were compiled by merging four databases: the ARC data registry, the Quebec Health Insurance Board database, the Quebec provincial database for hospitalizations, and the Quebec National Institute of Public Health database. A two-step cluster analysis was performed to generate distinct groups based on diagnostic categories of mental disorders. Complementary comparative analyses were conducted to test differences among the clusters. Results: Three clusters were identified. The first cluster consisted of individuals who did not receive any diagnosis of a mental disorder, including substance use disorders (SUDs), and who were low users of health services. The second cluster was composed of individuals who received a diagnosis of mental disorder, but not of SUD, and who primarily used mental health services. The third cluster included individuals who had a dual diagnosis (co-occurring SUD and mental disorder), and who were high users of acute care services. Conclusions: This study highlights the heterogeneity of individuals with problematic substance use treated in an ARC Treatment will be more effective, overall, if tailored to the varying needs of individuals with problematic substance use. Recommendations of the study geared toward improving the effectiveness of care for this population include systematic diagnostic screening for SUDs by physicians, developing strong primary care for individuals with problematic substance use, and ensuring a continuing care model for individuals with a dual diagnosis. (C) 2016 Elsevier Inc. All rights reserved.