Invasive versus non-invasive epileptogenic zone definition was analyzed in a series of 89 patients operated on for drug-resistant epilepsy. In the group of 69 cortical resections, 26% needed invasive recordings, 13.5% when foreign-tissue lesions had been detected by MRI and 32% when were absent. In this last group temporal resections had invasive EEG recordings in 23.5% versus 54.5% when the epileptogenic zone was extratemporal. In a group of 43 temporal resections with more than one year follow-up MRI has detected some abnormality in 84%. Excluding focal lesions, MRI detected hippocampal/temporal lobe atrophy in 66% of the cases in agreement with other noninvasive tests and in 4% contralateral to the epileptogenic zone located by subdural strips. The outcome analysis showed that 85% of the patients with MRI-EEG agreement were seizure free in contrast to only 43% when MRI was non-lateralizing. Future studies has to be oriented to better understand the epileptic process of patients without MRI abnormalities.