Introduction: What adolescents do to cope with emotional pain and negative affect is an important aspect of emotion regulation (ER), which can be predictive of adult mental health. As adolescents with severe ER problems are often referred due to suicidal ideation (SI), self-harm (SH) and emotional distress, treatments such as DBT-A focus on improving ER through behavioural change. Although prior studies have demonstrated DBT-A’s effectiveness in reducing SI and SH, such studies often rely on data collection that is sensitive to recall bias. Also, less is known about how such treatment in adolescence might affect behavioural aspects of ER into adulthood. Aims: Ecological Momentary Assessment was used to examine long-term outcomes related to emotion regulation in real-time. Methods: 55 young adults with a history of BPD and repeated DSH in adolescence, 12.4 years after participation in an RCT comparing DBT-A and Enhanced Usual Care (EUC), completed 1 week of EMA with 6 daily prompts. Daily and bi-hourly real-time fluctuations of positive and negative affect, self-harm, self-harm urge, death wish, suicidal ideation and coping strategies were collected. Results: Over the course of the observation period, 9% reported engaging in self-harm, 33 % reported self-harm urges, 22 % suicidal ideation and 44 % death wish. DBT-A was associated with fewer participants reporting SI, while there were no group differences in terms of SH, SH urge nor death wish. Participants who received EUC reported statistically significant higher levels of fear, whereas no differences were found on other affect measures. Across the whole sample, the most reported coping strategies were ‘keeping busy with activities’, ‘positive thinking’ and ‘socializing’, while the least common were ‘breathing exercise’ and ‘opposite action’. Conclusions: Young adults who received DBT-A as adolescents have lower probability of endorsing suicidal ideation and report lower levels of fear 12.4 years after treatment. Although reported self-harm is only present in 9 % of the sample, a third still report self-harm urges and close to half a death wish over the course of one week. These findings advance our understanding of how emotion regulation can be affected by specialized treatment in adolescence, and demonstrate the utility of using EMA to examine real-time behavioural outcomes.