We attempted to determine the significance of background abnormalities and their relation with spikes and location of atrophy in 56 patients with temporal lobe epilepsy (TLE) and mesiotemporal atrophy (MTA) assessed by volumetric magnetic resonance imaging (MRI): 35 patients had unilateral (group I) and 21 had bilateral atrophy with lateralized predominance (group II). Trains of delta waves over temporal regions were observed in > or = 90% of patients. They lateralized with accuracy equal to that of the spikes to the site of atrophy (delta 92% or 29 of 32 patients in group I and 63% or 12 of 19 of group II; spikes 85% or 28 of 33 of group I and 65% or 13 of 20 of group II). Delta waves and spikes occurred together in > 85% of cases. With respect to their location, a striking concordance was observed: delta activity almost always occurred ipsilateral with unilateral spiking (90% or 19 of 21 of patients with unilateral and 88% or 7 of 8 of patients with bilateral atrophy) and bilaterally independently with bilateral spiking (67% or 6 of 9 of patients in group I and 100% or 10 of 10 in group II). Furthermore, spiking and delta activity were never in disagreement with respect to lateralization. In TLE related to MTA, delta transients are a reliable indicator of the epileptogenic focus and presumably reflect the epileptogenic process rather than the underlying structural pathology.