Hemispherectomy is effective in arresting seizures associated with maximal or near maximal hemiparesis. This procedure, however, carries an unacceptable 33% risk of late complications due to cerebral hemosiderosis. Anatomically partial but functionally complete hemispherectomy was devised to avoid these complications. The frontal or occipital lobes, or both, were left in place with the blood supply intact but with connections to commissures and brainstem divided. The central strip and parietal and temporal lobes were removed. Twenty patients were so treated with a follow-up of 4 to 13 years (average, 7.3 years) in 14. Ten of these are seizure free, 1 had a single nocturnal seizure, 1 had occasional focal twitching, and 2 had a worthwhile but lesser reduction in the seizure tendency. None has developed cerebral hemosiderosis, to date. In appropriately selected patients, functional hemispherectomy is an effective procedure preferable to callosotomy or to partial hemispherectomy. When there is no independent ictal discharge from the opposite hemisphere, arrest of seizures may be expected, leading to improvement in cognitive functioning (mean increase, 10 IQ points), social behavior, and a reduction in or discontinuation of anticonvulsant medication. In these patients, gait and hand use remain unchanged.