The authors studied the lateralizing value for temporal epileptogenesis of focal or hemispheric EEG ictal features that first appear ≥5 seconds after hemispheric or diffuse EEG changes or after an artifact-obscured onset, by correlation with side of lobectomy abolishing or producing ≥90% improvement of complex partial seizures (CPS). One hundred forty-eight seizures in 39 patients rendered seizure-free (SF) by lobectomy lacked early localizing features of which 91 (61%) seizures in 37 (95%) patients had late lateralizing or localizing phenomena. Eighty-seven (96%) of 91 late localizing or lateralizing seizures occurred ipsilateral to lobectomy, involving 35 (95%) of 37 patients. Late contralateral and shifting phenomena occurred in a total of 4 seizures of 2 patients. Among 17 non-SF but ≥ 90% improved patients, 30 (48%) of 63 seizures lacking early localizing features showed late localizing or lateralizing phenomena. Of these, twenty (67%) involving 12 (71%) patients occurred ipsilateral to lobectomy. Among the remaining 5 (29%) patients, a total of 5 seizures (17%) were contralateral while 5 (17%) exhibited shifting laterality. These data indicate that seizures with late (≥ 5 seconds) localizing or lateralizing features cannot be discounted when assessing laterality of temporal epileptogenesis but cannot be used as the sole electrographic criterion.