It is generally held that the prescription of stimulants for hyperkinesis does not predispose children to later stimulant abuse. The case of a 13-year-old hyperactive boy who abused methylphenidate after two years of therapeutic treatment is reported in this paper. During a research hospitalization, he was administered methylphenidate in a double-blind placebo-controlled fashion to assess the mood changes induced by the drug and to verify expected therapeutic effects. The child met the research criteria of hyperkinesis and did demonstrate a therapeutic effect of methylphenidate. He did not, however, demonstrate the pattern of mood changes characteristic of adult stimulant users. Possible risk factors for later substance abuse in prepubertal patients on prescribed stimulant medications are discussed.