Objective: Methylphenidate has four optical isomers due to two asymmetries (erythro-threo and dextro-levo). The initial commercial formulation eliminated the erythro isomer, but the dextro-levo asymmetry was racemic, with equal amounts of d and l-threo isomers (d,l-MPH). Previous work has suggested that the d-threo isomer methylphenidate (d-MPH) rather than the l-threo isomer (l-MPH) is responsible for the clinical effects in children with attention-deficit/hyperactivity disorder (ADHD). This study compared the efficacy of acute equimolar doses of d-MPH and dl-MPH in reducing ADHD symptoms over an 8-hour period in a laboratory school setting and investigated the relationship of efficacy to plasma levels of MPH. Method: Thirty-two children with ADHD enrolled in this double-blind, placebo-controlled, crossover study, and 31 completed the study. On seven separate occasions separated by at least 6 days, the children received a single morning dose of d-MPH (2.5, 5, or 10 mg), d,l-MPH (5, 10, or 20 mg), or placebo and then were observed in a laboratory classroom setting for 8 hours. At specified intervals, blinded observers rated behavior, and the children performed a computerized math test. The plasma levels of MPH were related to the response to study medication. The safety profiles of the two formulations were compared. Results: For both formulations, the responses to both MPH preparations were dose related, the plasma concentrations of l-MPH were negligible and of d-MPH were indistinguishable, and clinical efficacy was highly correlated with plasma concentrations of d-MPH. The efficacy of the d-isomer was equivalent to the racemic preparation in reducing ADHD symptoms and increasing academic productivity. Conclusions: The efficacy of MPH resides in the d-isomer. The elimination of the l-isomer does not diminish the efficacy of an acute dose of methylphenidate.