We analyzed the spontaneous brain activity of a brain tumor patient during an epileptic seizure. The current density reconstruction by spatial filtering (CLSF) revealed pathologic, neuronal activity originating from the hippocampus ipsilateral to the tumor which was confirmed by intraoperative electrocorticography. This study shows that subcortical brain activity can be localized noninvasively using MEG. Further MEG studies combined with depth electrode recordings will have to show the validity of our results and might help to improve our understanding of the pathophysiology of epileptogenesis.