OBJECTIVES: The aim of this study was to determine the order of improvement in response inhibition, interference capacity, and processing speed in the Stroop test after starting methylphenidate treatment in children with ADHD. METHODS: This study included a total of 52 children aged 7–16 years who were diagnosed with combined-type ADHD for the first time and who began to use methylphenidate treatment. The Stroop test was applied to each subject at least 3 times (before treatment and at the first and second months of treatment) in the follow-up visits. RESULTS: The participants completed the fifth section of the Stroop test at a median duration of42.09 sec (quartiles: 35.58–54.0 sec) before treatment, while the median duration was 34.49 sec (quartiles: 27.43–34.48 sec) at the first month of treatment and 32.18 sec (quartiles: 26.97–32.18 sec) at the second month of treatment. The task completion duration showed a statistically significant improvement from the first month of treatment (p < 0.001). When the participants were compared in terms of the number of errors and corrections they made in the fifth section of the Stroop test, there was no significant difference between pretreatment measurements and post-treatment first month measurements (p > 0.05). The number of errors and corrections were statistically significantly lower in the second month of treatment compared to pretreatment and 1st mont of the treatment (p < 0.05). CONCLUSIONS: This study demonstrated that processing speed, response inhibition, and interference capacity assessed by the Stroop test improved with methylphenidate treatment in children with ADHD. This study is the first study to show that these improvements occur in a certain order over time.