Thirty-five children were tested with an auditory oddball P300 paradigm both before and 2 hr after a single-dose trial of methylphenidate (MPH). A prediction of the long-term benefit of medication was then made based on the magnitude of the acute changes in P3b amplitude. Those with postdrug amplitude increases of at least 30% were predicted to respond favorably to stimulants. All children were followed-up at 6 months. The original MPH Challenge Test predictions (based on acute post drug P3b amplitude changes) were then compared with the clinicians' evaluations of outcome. The MPH challenge classification accurately predicted outcome in 81% of cases.