A series of 47 patients studied with stereotactically implanted depth electrodes is analyzed. Indications and grouping of the patients fall into three main categories: (a) the bitemporal series (35 patients) where there is ambiguity as to the lateralization of the focus; (b) the unilateral series (6 patients) where the problem is one of localization within one hemisphere, and (c) the generalized series where one tries to determine the primary focus in secondary generalized seizures (6 patients). The best indication appears to be the bitemporal cases, in which a large number of attacks can be recorded by computer. Most of them tend to have a well-lateralized onset in one of the temporal lobes; 32 of the 35 implanted patients were operated upon (91.4%). The results on the seizure tendency in these patients is comparable, if not superior, to those obtained in the socalled unilateral cases. Correlation studies indicate that interictal activity is a good indicator of the predominance of seizures.