In the pediatric neuropsychiatric clinic of Ostfold Hospital Trust, Norway, patients diagnosed with attention deficit hyperactivity disorder are tested with quantitative EEG (QEEG) and ERPs to supplement the neuropsychological examination. In the majority of cases a systematic four weeks trial on stimulant medication is offered, and the clinical effects are evaluated at the end of the try-out period. This procedure is a clinical routine. In this study patients and parents were asked if they were willing to complete a second QEEG/ERP test on a single dose of stimulant medication shortly before the onset of the four weeks trial. They were explained that the aim of the study was to search for predictors of clinical response. They were also informed that this second test was completely voluntary. (In fact the vast majority were quite enthusiastic). The patients were characterized as responders (REs - medication continued) or non-responders (non-REs - medication stopped) based on explicit criteria. The single-dose induced changes in ERPs (and behavioral parameters from the attention test; number of errors, reaction times) were examined, and differences between REs and non-REs were calculated.