In this report 17 patients with long-standing non-focal epilepsy underwent callosotomy (this was total in two patients and performed in two stages, and anterior-subtotal in the remaining patients). In all patients the atonic-hypertonic seizures with sudden falls were the most disabling epileptic fits. Callosotomy proved efficient in controlling atonic fits in 10 out of 15 patients in whom surgical results are evaluated. In 3 additional patients the frequency of atonic fits was reduced by more than 50%. In the remaining two patients, no therapeutic effect was observed. Callosotomy was less effective on seizures which were not atonic. Therefore, this procedure appears to be indicated in patients in whom atonic fits are predominant. The main effect of callosotomy is to transform drug-resistant seizures into drug-sensitive ones. Neuropsychological sequels are insignificant unless the splenium is severed. However, considerable psychic and behavioral improvement was nearly always observed after surgery. Despite the fact that on a therapeutic level results were often satisfactory, a number of practical problems still remain. These concern the full spectrum of indications for callosotomy, the extent of corpus callosum section, choice of methods in severely mentally retarded patients and, finally, the age at which the operation should be carried out.