Methylphenidate is an effective and well tolerated agent frequently used in the treatment of attention deficit hyperactivity disorder. Thrombocytopenia is defined as a blood platelet count less than 150,000/mm(3), and is an important hematological side-effect capable of causing mild symptoms or severe complications such as intracranial hemorrhage. Thrombocytopenia may develop in association with various causes, including drug use. There have been very few case reports of thrombocytopenia developing during methylphenidate use, and to the best of our knowledge there have been no reports concerning the management of this side-effect. This report describes the development of thrombocytopenia likely related to methylphenidate therapy in an adolescent with attention deficit hyperactivity disorder and successful management with a safety switch to atomoxetine.