This study compares ictal semiology, neurological examination and scalp EEG be tween lateral and mesial occipital epilepsy to assess the contribution non-inva sive data make in determining the epileptogenic region within an occipital lobe. We assessed seizure origin in 41 occipital patients as lateral (11 patients), m esial (20) and both surfaces (10) as indicated by subdurally recorded seizures ( nine), a lesion whose removal reduced seizure quantity by ≥90%(six), or who me t both criteria (26). No aspect of semiology distinguished lateral from mesially originating occipital seizures. A pre-operative visual field deficit appeared in eight (42%) out of 19 testable patients with mesial originating seizures, th ree (30%) out of 10 patients with both surfaces epileptogenic, but none of the 10 testable patients whose seizures arose only from the lateral surface (P=0.037 3, lateral versus mesial and both surfaces). Although occipital seizures appeare d on the majority of the first five scalp EEG recordings in four (36%) out of 1 1 patients with laterally originating occipital seizures compared with none of 2 0 patients in whom seizures originated mesially (P=0.0105), no other scalp EEG f eature distinguished seizures from these surfaces. We conclude that subdural ele ctroencephalography is likely to be necessary to delineate the epileptogenic reg ion within an occipital lobe. Nonetheless, focally originating scalp-recorded s eizures accurately lateralized the epileptogenic zone in 20 (49%) of our 41 pat ients compared with only one (2%) which originated contralaterally (P=0.0001). This relationship held when considering only the first five scalp EEGs: the seiz ures of 10 patients (24%) appeared ipsilaterally and none contralaterally (P=0. 001). Moreover, interictal occipital (01,2) and posterior temporal (T5, T6) spik es appeared consistently and significantly (P < 0.001) more commonly ipsilateral to epileptogenesis than contralateral using multiple methods of analysis. This study compares ictal semiology, neurological examination and scalp EEG be tween lateral and mesial occipital epilepsy to assess the contribution non-inva sive data make in determining the epileptogenic region within an occipital lobe. We assessed seizure origin in 41 occipital patients as lateral (11 patients (es) (20) and both surfaces (10) as indicated by subdurally recorded seizures (nine), a lesion whose loss of reduced seizure quantity by ≥90% (six), or who me t both criteria aspect of semiologically distinguished lateral from mesially originating occipital seizures. A pre-operative visual field deficit was found in eight (42%) out of 19 testable patients with mesial originating seizures, th ree (30%) out of 10 patients with both surfaces epileptogenic, but none of the 10 testable patients whose seizures arose only from the lateral surface (P = 0.037 3, lateral versus mesial and both surfaces). Although occipital seizures appeare d on the majority of the first five sc alp EEG recordings in four (36%) out of 1 1 patients with laterally originating occipital seizures compared with none of 2 0 patients in whom seizures originated mesially (p = 0.0105), no other scalp EEG etystic distinguished seizures from these surfaces. We Conclude that subdural ele ctroencephalography is likely to be necessary to delineate the epileptogenic reg ion within an occipital lobe. Nonetheless, focally originating scalp-recorded s eizures accurately lateralized the epileptogenic zone in 20 (49%) of our 41 pat ients compared with only one (2%) which originated contralaterally (P = 0.0001). This relationship held when considering only the first five scalp EEGs: the seizures of 10 patients (24%) was ipsilaterally and none contralaterally (P = 0.001) Interictal occipital (01,2) and posterior temporal (T5, T6) spikes were consistently and significantly (P <0.001) more commonly ipsilateral to epileptogenesis than contralateral using multiple methods of analysis .