PURPOSE: To identify the clinical and electroencephalographic factors which are independently predictive of a postoperative seizure-free outcome for 4 years. We compared the outcomes of the first 2 years with the subsequent 2 years one after anterior temporal lobectomy (ATL) for mesial temporal lobe epilepsy (MTLE) with unilateral hippocampal atrophy (HA) on MRI. METHODS: We studied 51 consecutive operated patients who had above 4 years of follow-up and had MTLE with definite unilateral HA on MRI. The surgical outcome was classified as either seizure-free or not seizure-free in the first postoperative 2 years and the subsequent 2 years. Several clinical variables were included. The scalp EEG parameters included the lateralization of interictal epileptiform discharges, ictal onset location, ictal onset frequency, ictal EEG lateralization, and ictal scalp EEG propagation (bitemporal asynchrony or switch of lateralization). Variable factors were subjected to univariate analysis. RESULTS: Overall, 36 patients (71%) became seizure-free during the postoperative 4 years. On univariate analysis, only one factor was significantly associated with poor outcome (p<0.05): ictal scalp EEG propagation pattern such as bitemporal asynchrony or switch of lateralization. The seizure-free outcome was seen in 88.9% of patients without bitemporal asynchroncy, or switch of lateralization while only 54.5% of patients with those patterns (p=0.007) during the postoperative third and fourth year. However, those propagation patterns did not show the prognostic value during the first 2 years (p=0.449). Other variable factors were found not to be predictive of prognosis on early or late recurrence. CONCLUSIONS: Bitemporal asynchrony or a switch of lateralization in the ictal scalp EEG might be a highly predictive factor for an undesirable surgical outcome, late recurrence of seizure during a follow-up period after ATL, and probably an index of bitemporal epileptogenicity in MTLE.