To assess the clinical value of magnetoencephalography (MEG) in investigating the origin of secondary bilateral synchrony (SBS) in patients with partial epilepsy. MEG and simultaneous electroencephalography (EEG) were recorded with a 204-channel whole-head MEG system in 2 patients. The equivalent current dipoles (ECDs) for epileptic discharges on MEG were calculated according to a single dipole model. In patient 1, the ictal EEG showed bursts of bilateral synchronous 3-Hz spike-and-slow-wave complexes. ECDs obtained from the ictal MEG localized to the right medial frontal lobe. On the second patient's MEG recordings, epileptic discharges corresponding to prolonged EEG bursts of bilateral synchronous spike-and-slow-wave complexes were obtained. ECDs calculated from the prolonged bursts were clustered in the left medial frontal lobe. MEG detected the sources of SBS in the medial frontal lobe. MEG is extremely useful for the identification of the source of SBS.