Using spike-triggered fMRI, we sought to document regional changes in blood oxygen level-dependent (BOLD) activity associated with spontaneous generalised spike and slow-wave discharges (S&W). Five adult patients were studied who had idiopathic generalised epilepsy (IGE) and frequent S&W. EEG was recorded inside a 3T MRI, allowing acquisition of single, whole-brain fMRI images following S&W, as well as baseline images. Between 4 and 25 spike and wave complexes were captured in individuals. Four of the five individuals showed significant S&W-related BOLD signal reductions ("deactivation") in the posterior cingulate (P<0.001 uncorrected). Significant changes were absent only in the individual with fewest spikes. Group analysis including all five subjects confirmed fMRI deactivation in the posterior cingulate. Some S&W-related BOLD signal increases were seen in the depths of the precentral sulci in individuals and on group analysis. No significant S&W-related changes in thalamic BOLD activity were observed. The posterior cingulate may have a role in the electroclinical phenomenon of S&W and "absence." Possibilities include a causative role, with reduced activity in the cingulate facilitating the onset of S&W, a secondary role, explaining the cognitive changes observed during prolonged S&W, or an epiphenomenon.