Using magnetoencephalography (MEG), we evaluated epilepsy patients who displayed a widespread distribution of electroencephalography (EEG) spikes. The propagation of intra- and inter-hemispheric interictal spikes was measured using MEG in 9 cases. Five cases with continuous spikes and waves during slow sleep (CSWS) showed inter-hemispheric propagation that suggested secondary bilateral synchrony. Three of the four patients with localization-related epilepsy (LRE) showed intra-hemispheric, anterior–posterior or posterior–anterior propagation, with one patient with LRE exhibiting inter-hemispheric propagation. MEG was used to measure the propagation of interictal spikes from epilepsy patients who displayed a widespread distribution of spikes by EEG. MEG may thus be a useful tool for determining the true epileptic focus in these patients.