Objective: We developed a novel non-invasive analysis to localize the source and visualize the time course of electrical activity generated inside the brain but unclear from the scalp. This analysis applies to signals with unique waveform characteristics, such as seizures. Methods: The method extracts activity from an EEG data matrix as a spatiotemporal component having waveforms uncorrelated to the other concurrent activities. The method also provides the location and orientation of the dipole generating this activity. We applied this method to ten scalp seizures in three patients with temporal lobe epilepsy and single-focus seizures confirmed by intracerebral recordings. A realistic head model based on MRI was used for computation of field distributions. Results: When seizure activity was still not visually identifiable on the scalp, the method demonstrated in all scalp seizures a source in the temporal neocortex corresponding clearly to the region of seizure activity in intracerebral recordings. Frequency characteristics of the estimated activities also resembled those of the intracerebral seizures. Conclusions: This method enables estimation of focal brain activity when its effect on scalp EEG is unclear to visual examination. It works in situations where currently available source analyses methods, which require noiseless definite activity, are not applicable. Copyright (C) 2000 Elsevier Science Ireland Ltd.