Background and Aim Emotional dysregulation (ED) is a frequent feature of attention-deficit/hyperactivity disorder (ADHD). It can be observed as a dysregulation profile or a deficient emotional self-regulation (DESR) profile. Oppositional defiant disorder/conduct disorder (ODD/CD) comorbidity is prevalent in ADHD and known to be related with ED. The first-line treatment of ADHD includes psychostimulants, but their effects on ED are not well studied. This study aimed to evaluate the outcomes of methylphenidate (MPH) treatment on ED in ADHD + ODD/CD cases. Methods A total of 118 ADHD + ODD/CD patients with a mean age of 9.0 1.9 years were treated with MPH for 1 year. Also, parents of cases were recruited for a parent-training program, which initiated after first month of MPH treatment. Symptom severity was assessed at baseline and 12th month by Turgay Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition-Based Child and Adolescent Behavior Disorders Screening and Rating Scale-Parent Form, Children Depression Inventory, Child Behavior Checklist 4-18 years, and Parental Acceptance and Rejection Questionnaire-Mother Form. Results Emotional dysregulation (DESR + DP) was present in 85.6% of cases. Conduct disorder was significantly higher in patients with DP, whereas ODD was significantly higher in the DESR and non-ED groups (P < 0.0001). Symptoms of ADHD and ED were significantly improved with 1-year of MPH treatment (P < 0.05). The improvement in ED was independent of improvement in ADHD symptoms and parent training (P < 0.05). Conclusions Emotional dysregulation is highly prevalent in disruptive behavioral disorders as ODD and CD, which are comorbid with ADHD. The MPH treatment is effective on ED independently from other clinical determinants.