We report the results of 34 patients who underwent corpus callosotomy between 1986 and 1989 with 28-65 months of postoperative follow-up (mean 42 months). Thirty-two patients had mental retardation and 26 had significant behavioral problems. Thirteen patients had total section, 8 had subtotal section with preservation of the posterior half of the splenium, and 13 had section of the anterior two thirds of the callosum. Satisfactory seizure control was achieved in 25 patients (73.5%) Atonic seizures, followed by tonic seizures, generalized tonic-clonic seizures (GTCs), and atypical absence seizures were most improved. Myoclonic and complex partial seizures (CPS) did not improve significantly. No deterioration in seizure status was observed postoperatively. Two patients developed previously unobserved simple seizures and CPS postoperatively, but they were not as disabling as the preoperative seizures. Among the patients with behavioral problems, 81% had significant decrease in aggressiveness, hyperactivity, and/or attention deficit. Patients who underwent total section had interhemispheric disconnection symptoms that improved progressively and did not interfere with daily life. Decreased speech output, dysarthria, and gait dyspraxia occurred after total callosal section and persisted in 5 of the 13 patients. Patients who underwent anterior two thirds or subtotal sections did not have such symptoms. Early postoperative complications consisted of aseptic ventriculitis (5), subdural hematoma (1), and wound infection (4) and resolved without sequelae.