Long-term subdural electroencephalographic (EEG) recording was performed in a series of patients with medically intractable complex partial seizures to test the hypothesis that ictal interhemispheric propagation time (IHPT) is correlated with temporal lobe epileptogenicity. In 41 patients, the duration from initial subdural EEG seizure onset to the first appearance of subdural EEG epileptic activity in the contralateral hemisphere (IHPT) was measured in seconds and analyzed for a quantitative relationship to temporal lobe seizure interval (frequency⁻¹), in hours. A statistically significant, nonlinear correlation between IHPT and seizure interval was found (Arctan y=-0.009x²+0.598x+75.187, y=IHPT, in seconds, x=seizure interval, in hours, r=0.326, d.f.=39, t=2.15, p<0.05). The results suggest that, for seizure intervals less than 33h, increasing IHPT is associated with increasing seizure interval (i.e. decreasing epileptogenicity). Conversely, for seizure intervals greater than 33h, decreasing IHPT is associated with increasing seizure interval. Because the relationship between IHPT and seizure interval is a trigonometric (i.e. arctangent) function of a second degree polynomial, small changes in IHPT are associated with substantial changes in seizure interval. These findings suggest that temporal lobe epileptogenicity is a complex, nonlinear function of the electrocorticographic EEG time factor (i.e. IHPT) involved in the transmission of ictal epileptic activity from the seizure focus to the contralateral hemisphere. The results suggest that, on an electrophysiologic basis, patients with temporal lobe epilepsy represent two distinct populations based on seizure interval. The results should improve the understanding of the electrocorticographic pathophysiology of temporal lobe epilepsy. The development of cortical neuromodulation strategies designed to suppress temporal lobe seizures should consider this complex relationship between temporal lobe interhemispheric propagation time and epileptogenicity.