In 36 patients with drug-resistant epilepsy submitted to anterior callosotomy (27 cases), to two-stage total callosotomy (8 cases) and to posterior callosotomy (1 case) the EEG variations concerning background activity, focal activity and sharp-waves (SW) bisynchronous activity were evaluated. EEG modifications observed after callosotomy are the following: background rhythm tends to be better organised as spectral analysis demonstrated, this finding usually coincide with reduction of bisynchronous discharges. It appears that improvement in background activity cannot be correlated with outcome, but it seems to be to some extent since at the same time cognitive functions also seem to improve; however, this last aspect need to be checked in much larger series. The number and location of EEG foci do not change, but they appear to be more active; this is likely to depend only on the concomitant reduction of bisynchronous activity. No correlation seems to exist between the number and the location of foci, which are generally multiple. Lateralization of bisynchronous discharges as well as the reduction of their frequency and duration were observed. However, the clinical course is quite different: in some patients we have achieved good clinical responses in others postoperative results were poor. Lateralization of bisynchronous discharges is never absolute, on the grounds that in prolonged recordings bisynchronous discharges are nearly always present. Bisynchronous discharges in some cases are alternatively predominant in both hemispheres even within minutes or seconds. It was observed that after certain time, generally some months, lateralized discharges tend to generalize again, confirming that corpus callosum is replaced in discharge diffusion by other structures (brain-stem, diencephalon).