This review of the effects of hemidecortication in humans has been limited to studies of cognitive outcome published during the last 20 years. More directly than in the case of split-brain patients, the patients reviewed here attest to the remarkable ability of a single hemisphere, whether left or right, to support at least at modest levels a wide range of cognitive functions--from visual perception, through memory and intellectual processes, to language and even speech. In some cases, as has been indicated, the surgical removal of a diseased hemisphere has resulted in improvement of cognitive function. This positive outcome has occurred more frequently in patients with early (i.e. congenital or perinatal) onset of seizure disorders than in those with late onset (e.g. Rasmussen's disease). But even for the latter patients, the cognitive costs of the surgery per se have rarely been severe. And in both types of case, the incidence of either complete or substantial postoperative relief from intractable seizures has been high, ranging around 80-90%. Although the therapeutic efficacy and small cognitive costs of the surgery are now quite well established, little is known yet regarding the specific cognitive defects that arise from the loss of one as opposed to the other cerebral hemisphere. Intelligence levels have been found to be equally low, averaging in the mid-60s and almost never rising above 100 in patients with either left or right hemidecortications, and memory quotients have most often appeared to fall in line with the IQ scores, again without clear evidence of any difference in the effects of left and right removals. Even in the case of visual spatial perception, considered to be a hallmark of right hemisphere function, the evidence is unclear, one study reporting selective impairment on difficult visuospatial tasks in right hemispherectomised patients, but another not. Only in regard to language processes is there a consensus regarding the differential effects of left and right hemidecortication, and here the differences are apparent only in the relatively subtler aspects of language. Thus, the isolated right hemisphere is at a significant disadvantage compared with the left in the comprehension of abstract, low frequency words, in phonetic feature analysis, and in the subtleties of grammar, such as the comprehension of passive negative constructions and the correct use of morphological markers in unfamiliar contexts (e.g. application of comparative and superlative forms of an adjective to nonwords.(ABSTRACT TRUNCATED AT 400 WORDS)