OBJECTIVE: The authors examined the effects of chronic stimulant treatment on cerebral glucose metabolism in adults diagnosed with attention deficit hyperactivity disorder (ADHD), who were studied by means of positron emission tomography (PET) with [18F]fluorodeoxyglucose as the tracer. METHOD: Each subject received two PET scans, the first before drug treatment and the second after treatment with daily oral doses, individually titrated for clinical effect, of either methylphenidate (N = 19) or d-amphetamine (N = 18) for a minimum of 6 weeks. The subjects completed behavioral self-report measures before and at the end of the medication period. RESULTS: Neither stimulant medication changed global, or whole-brain, metabolism, although both drugs increased systolic blood pressure. Metabolism in only two of the 60 brain regions sampled was changed by methylphenidate, while d-amphetamine exhibited no effect on regional metabolism. Both drugs were associated with significant improvement in behavior, as evidenced by improved ratings for restlessness and ability to maintain attention. CONCLUSIONS: While the present study does not demonstrate any robust metabolic effects of chronic stimulant treatment, the behavioral data strongly indicate that methylphenidate and d-amphetamine are effective agents for the treatment of adults with ADHD.