Fifty patients with medically refractory partial seizure disorders have undergone subdural electrode placement for seizure focus localization. Standard scalp telemetry recordings of ictal events had failed to demonstrate accurately the site of seizure onset, and these patients were considered candidates for telemetry with intracerebral depth electrodes. Excellent recordings of interictal and ictal events were obtained, and localization of the epileptogenic focus was derived from recordings made during spontaneously occurring seizures. Electrocorticograms were monitored for up to 21 days. The recordings enabled a surgical decision to be made in 43 of 50 cases (86%). Thirty patients have subsequently undergone cortical excision of their foci with good results. Subdural electrode recordings are a significant addition to the armamentarium of the neurosurgeon attempting to localize surgical seizure foci, offering a low morbidity procedure as an alternative to depth electrode implantation.