Two infants are described with intractable, drug-resistant seizures from birth associated with an enlarged cerebral hemisphere. The first died at sixteen months and demonstrated arrested head growth. Histology of the abnormal hemisphere showed disturbed cortical architecture, and subcortical heterotopias with multinucleate cells resembling tuberous sclerosis cells. Continuous fitting persisted in the second infant following callosal section. Hemispherectomy was undertaken in an attempt to preserve function of the "normal" hemisphere. Seizures ceased and head growth velocity accelerated. The neuropathology of the excised hemisphere is compared with the first case. From the literature this is the first report of a favourable outcome after hemispherectomy for hemimegalencephaly. The finding of an echodense enlarged hemisphere may allow presumption of this diagnosis in a neonate with unilateral seizures. After CT scan the diagnosis may be confirmed histologically providing that a precisely orientated frontal biopsy is submitted to the neuropathologist. We suggest that very early hemispherectomy may be the treatment of choice for this condition.