A group of 10 patients suffering from tuberous sclerosis (TS) and epilepsy was studied by means of topographic mapping of EEG and visual evoked potentials. The localizing ability of the technique was compared to the topography of hyperintense T2-dependent areas seen on magnetic resonance images (MRI). Data were collected from 19 electrodes, free from interictal EEG transients and artifacts, spectral analysis was performed using the FFT algorithm and color maps were produced by specialized equipment. Data were analyzed with respect to the presence of interhemispheric asymmetries and significant differences with age-matched normal controls. The topography of slow frequency components was the best clue for localization, with a concordance between imaging and spectral EEG data in 76% of the lesions detected by MRI. In 7 patients topographic mapping revealed spectral abnormalities in areas where MRI was not able to detect morphological lesions. These abnormalities were characterized by an increased delta (5 patients), theta (4 patients), or lowered ipsilateral alpha (1 patient) power. Although full agreement between imaging and electrophysiological data was not observed, we believe that in patients with TS the study of EEG rhythm activity can add valuable information to the visual inspection of the tracings.