Growth deficit associated with stimulants in children who have attention deficit hyperactivity disorder (ADHD) has long been the subject of scientific discussion. More recent studies describe the effects on the final height of children with ADHD who are treated with methylphenidate (MPH) as only slight. There are only a handful of reports of severe growth deficits attributed to MPH that are associated with gastrointestinal side effects. We describe a 10-year-old boy with ADHD and chronic asthma who underwent corticosteroid therapy and developed an almost complete growth arrest during MPH treatment. Growth hormone (GH) stimulation tests and measurement of GH-dependent growth factors point to the influence of MPH on GH secretion with subsequent impaired growth. One may conclude that some children are at risk of serious growth decrement when treated with MPH. The growth of children should thus be monitored carefully, even if there are no alarming gastrointestinal side effects from MPH. We found that the determination of growth velocity was a sensitive marker for the evaluation of growth impairment in our patient.