Stuttering and attention deficit hyperactivity disorder (ADHD) can be seen together. Anemia or thrombocytopenia, rarely even pancytopenia may occur as a side effect of medications used to treat both disorders. Thrombocytopenia, although it may be seen in some cases using methylphenidate, occurs rarely. An 8 year-old boy was brought to our outpatient clinic by his family with the complaints of stuttering, attention deficit, and hyperactivity. After the psychiatric evaluation and history were conducted and psychometric tests were applied. One month later methylphenidate 18mg/day was started for the treatment of ADHD. Soon after initiation of medication, petechia developed on both lower extremities of the patient. CBC showed isolated thrombocytopenia and te patient was followed by hematology clinic. On the 6th day upon stopping mehtlyphenidate, the thrombocyte count returned to normal. We also discussed possible mechanisms of thrombocytopenia.