Background: Young people with substance use problems face a high risk of co-occurring mental health problems, something that may involve a more difficult life situation, social problems as well as worse treatment outcomes. The aim of this study is to analyse self-reported mental health problems among young people receiving outpatient treatment for substance use problems in Sweden. We explore what types of mental health problems are more or less predominant, and whether there are significant differences between boys and girls. In addition, we analyse how various mental health problems covary with indicators of substance abuse severity. Methods: The study is based on structured interviews with 1970 young people enrolled at outpatient clinics in 11 Swedish cities. The data was analysed through frequency-and averages-calculations, Chi-square tests and multivariate logistic regression analyses. Results: Self-reported mental health problems were common among the young people in the study. A relatively large percentage of the total group (34-54%) reported problems such as concentration difficulties, sleeping difficulties, anxiety and depression. At the same time, many of the young people did not report any symptoms and only a small group, about 20%, reported diagnosed mental health disorders. The results show substantial gender differences, with girls reporting significantly higher levels of mental health problems. Multivariate logistic regression analyses demonstrated significant associations between severity of drug use problems and anxiety, concentration difficulties, aggression, hallucinations and mental stress caused by experiences of trauma. Conclusions: Treatment needs are diverse within this group of young people who use drugs. Since girls report higher levels of all mental health problems, and a larger burden of psychosocial risk factors than boys, they are likely to require more comprehensive treatment interventions. The link between more severe drug problems and mental health problems points to the importance of exploring this relationship in treatment. A multidisciplinary approach, in which co-occurring problems can be addressed simultaneously, may be the best treatment form for many young people with drug problems.