About one third of manic patients show a decreased Electroencephalographic Vigilance Level (EVL). In two patients with recurrent manias the hypothesis was tested that the EEG may be used to predict the therapeutic response to methylphenidate. Patient A (with a decreased EVL) responded rapidly to methylphenidate with a considerable reduction of symptoms and a clear increase of EVL, whereas patient B (normal EVL) did not show any change, neither clinically nor in the EEG.