Patients (28) were studied with intractable seizures in whom exclusively extratemporal or a combination of temporal and extratemporal electrodes were chronically implanted for the localization of the epileptogenic process and possible surgical treatment. Clinical and electrographic data were briefly summarized, the indications for the use of implanted electrodes were outlined and the number and position of the electrodes and the findings in each individual case were given. Four illustrative examples were described in greater detail. Mainly on the basis of data derived from this particular technique of investigation, surgical treatment was eventually carried out in 14 patients. The use of implanted electrodes in seizure disorders of probable extratemporal origin can be of real diagnostic benefit in certain specific situations. In most instances, however, this technique simply served to demonstrate the complexity of an apparently simple case or, of greater clinical consequence, might tend to oversimplify cases that are actually very complex. Indeed, many data in this study raise some doubts about the validity of the classic concepts of focal epilepsy.