SYNOPSIS We report for the first time the detection by magneto-encephalography (MEG) of signals observable inmigraine patients during headache, but not in controls. These signals consisted of three features: suppression of spontaneous cortical activity, long duration field changes, and large amplitude waves(LAW) of several seconds duration. LAW were also seen during the interictal period. We discuss the possible relationship of these signals to spreading depression (SD), and why the LAW have not been observed in previous studies of SD.