12 cases, aged 8 mo.-21 yrs., of infantile hemiplegia complicated by convulsive and mental changes were hemispher-ectomized. The thalamus, the caudate nucleus and its tail and the cortex medial to the tail of the caudate nucleus were spared, the remainder of the hemisphere being removed. There was 1 fatality. Post-operative recovery was rapid with lessening of spastic athetosis and recovery of the ability to walk. There is at first profound loss of sensory modalities, but sensibility to pain and tactile stimulation (normal by comparison with the opposite side) returns in a few mos. A permanent contralateral homonymous hemianopia with macular sparing was induced, where not preoperatively present. Personality change from an asocial, ill-tempered and hyperemotional makeup to a good natured, tractable, cooperative disposition was the general rule. The ipsilateral hemisphere is interpreted to have some control over the volitional act, via the contralateral caudate nucleus. The non-dominant hemisphere may have been removed in all cases.