Purpose – The purpose of this paper is to assess whether mental health symptoms affect one-year reoffending rates upon release from prison for participants engaging in substance dependence treatment in the UK. Design/methodology/approach – A retrospective cohort study was used to assess reconviction outcomes upon release. The Comprehensive Addiction and Psychological Evaluation (CAAPE) was administered to 667 inmates admitted to the programme. The effect of mental health, drug use,and static risk factors on reoffending was assessed at one-year post release. Findings – Logistic regression analysis showed that symptoms of Major Depressive Disorder at the start of substance dependence treatment increased the likelihood to reoffend, whilst Obsessive Compulsive Disorder symptoms and length of sentence decreased the likelihood to reoffend. Antisocial Personality Disorder symptoms show a trend towards increasing the likelihood to reoffend. In addition, previously established risk factors for reoffending, including dependence on heroin, crack/cocaine, and poly drug use significantly increased the likelihood of reconviction. Practical implications – Depressive symptomatology pre-treatment could affect reoffending outcomes for participants in substance dependence treatment in prison. An integrative approach addressing both substance misuse and mental health factors is pivotal. Future efforts to address both simultaneously can be made to improve assessment, training, treatment, and through care for prisoners in substance dependence treatment. Originality/value – Few studies have assessed the effect of mental health factors on reoffending outcomes for offenders in substance dependence treatment. A large sample was studied in an understudied population of UK prisoners in substance dependence treatment. The results have implications for clinical settings where mental health symptoms are not addressed concurrently with substance dependence. This finding can inform policy makers and practitioners who provide substance dependence treatment in prison.