© 2019 Association for Child and Adolescent Mental Health Background: Dialectical behaviour therapy for adolescents (DBT-A) is an intervention with a growing evidence base for treating adolescents with emotional and behavioural dysregulation. Previous studies have reported on varying lengths of treatment, however, and optimal treatment duration has not yet been identified. While the treatment developers initially proposed a 16-week programme, they have more recently recommended an extension to 24 weeks. This study compares outcomes for adolescents and parent/guardians who participated in 16- and 24-week DBT-A programmes in a community setting. Methods: Eighty-four adolescents and 100 parent/guardians participated in 16-week DBT-A, while 68 adolescents and 67 parent/guardians participated in the 24-week programme. Outcome measures for adolescents included the presence and frequency of self-harm, suicidal ideation and depression; and for parents were burden, grief and parental stress. Outcomes were assessed at pre- and postintervention. Linear mixed-effects models were used to estimate the treatment duration effect (24-week vs. 16-week) utilising all available data at pre- and postintervention. Results: Data analyses showed a reduction in the presence and frequency of self-harm at postintervention for adolescents in both programmes. Both adolescent and parent participants in the 16- and 24-week programmes also showed changes indicating significant improvement on all self-report outcome measures (p <.05). A treatment duration effect was identified with adolescents in the 24-week programme reporting greater gains on measures of suicidal ideation and depression (p <.05). However, drop-out rates were higher for the 24-week programme. Conclusions: The findings of the current study indicate that 24-week DBT-A may have additional benefits in comparison to 16-week DBT-A in terms of further reductions in suicidal ideation and depression. Given the nature of this study, it was not possible to explore a potential time effect, however, so these results should be interpreted with caution. Further research will assist in determining an optimal programme duration of DBT-A.