Attention deficit hyperactivity disorder (ADHD) is frequently comorbid with borderline personality disorder (BPD). However, few studies have examined how comorbid BPD-ADHD patients, treated or not with methylphenidate (MPH), respond to psychotherapy compared to non-comorbid BPD patients. In this perspective, we used a naturalistic study to compare, during a month-long intensive dialectical behaviour therapy (DBT), the clinical course of BPD patients and comorbid BPD-ADHD patients who were treated or untreated with MPH. Out of the 158 BPD patients recruited, 59 had adult ADHD as a comorbidity; among these, 29 underwent a treatment with MPH or des-methylphenidate, while the 30 others did not. MPH treatment was given non-randomly and only when ADHD was considered to be hampering the capacity of the subjects to follow the therapy. Patients completed the following forms upon admission and after 1 month of treatment: the adult ADHD Self-Report Scale (ASRS v.1.1), the Barratt Impulsiveness Scale (BIS-10), the State-Trait Anger Expression (STAXI), the Beck Depression Inventory II (BDI-II), and the Beck Hopelessness Scale. At baseline, comorbid BPD-ADHD patients showed significantly higher impulsiveness than BPD patients. In the entire sample, there was a significant decrease in all dimensions ranging from small to large effect sizes during the 4-week intensive DBT. BPD-ADHD patients who were undergoing MPH treatment showed a significantly improved response to DBT treatment for Trait-State Anger scores, motor impulsiveness, depression severity, and ADHD severity, when compared to those without stimulant medication. This study outlines the importance of systematically screening BPD patients for ADHD, since a MPH-based treatment will improve the symptoms of patients who are comorbid for BPD and ADHD. Due to the non-random allocation of subjects, more severely affected patients were more readily placed on MPH; this suggests that the more severe the ADHD symptoms, the greater the chance for the patient of being treated.