The present study examined mental health symptoms and their relationship to cannabis use and treatment outcomes in a sample of adolescents who received treatment for cannabis dependence through a residential substance use program. The sample included 132 adolescents who nominated cannabis as their primary drug of concern upon admission and who completed at least 30 days of treatment. This study found that mental health symptoms of young cannabis users reduced significantly from admission to three-month follow-up. Further, pretreatment symptoms of anxiety were associated with greater pretreatment cannabis use, while symptoms of phobic anxiety were associated with less pretreatment cannabis use. Pretreatment obsessive-compulsive and somatization symptoms were associated with greater follow-up cannabis use, whereas pretreatment paranoid ideation symptoms were associated with less follow-up cannabis use. Further, follow-up somatization, obsessive-compulsive symptoms, hostility, and phobic anxiety were associated with greater follow-up cannabis use while follow-up symptoms of interpersonal sensitivity were associated with less follow-up cannabis use. These findings highlight a variety of areas for further investigation in order to enhance current treatment for cannabis use.