Objective: Stimulants are safe and effective medications for the treatment of ADHD. There are a number of case studies that report stimulant-induced dyskinesia. The aim of this study was to compare dyskinesia in a treated and a treatment-naive group of children with ADHD, and a healthy control group. Method: Children aged 6 to 18 years were involved in the study (n = 158). Diagnosis of ADHD was measured with the Mini International Neuropsychiatric Interview Kid (MINI Kid). Dyskinesia was assessed with the Abnormal Involuntary Movement Scale (AIMS). Results: Before methylphenidate administration, the treated ADHD group showed significantly higher AIMS total score than the control group (p = .001) and the treatment-naive ADHD group (p < .001). We found the same pattern 1.5 hr after methylphenidate administration. Conclusion: These results call attention that clinicians should take special care for the possible development of dyskinesia during the treatment of their ADHD patients with methylphenidate.