Two studies assessed the value of temporal lobe interictal electroencephalographic (EEG) spikes and delta in indicating side of temporal epileptogenesis. The first study determined laterality of spikes/delta in awake recordings of 56 patients whose seizures all began unilaterally as proven by (1) EEG-recorded seizures and (2) > 90% improvement after lobectomy. Spikes of 52 (93%) and delta of 46 (82%) patients predominated or appeared exclusively ipsilateral to seizure origin. Neither predominated contralaterally in any patient. The second study investigated laterality of temporal seizures in a separate group of 156 patients with various side vs side spike or delta ratios on 1 to > or = 4 awake recordings. Ninety-nine of 104 patients (95%) with temporal spikes on four or more awake recordings had most or all seizures ipsilateral to most spikes, including 79 of 80 (99%) of those with > or = 3 side vs side spike ratios. Among the 120 patients with high (> or = 3) side vs side spike ratios, most or all seizures of 118 (98%) originated ipsilateral to most spikes. Predominant seizure origin also correlated with lateralized arrhythmic delta--from 90% ipsilateral seizures of those with one EEG with delta to 100% with > or = 4 such EEGs. Data from these two studies using opposite directions of analysis (seizures-->spikes/delta and spikes/delta-->seizures) demonstrate high correlations between laterality of interictal and ictal entities, particularly if temporal spikes clearly predominate on one side and if unilateral temporal delta activity persists over several recordings. Such correlations suggest that the awake interictal scalp EEG cannot be ignored when assessing laterality of temporal epileptogenesis.