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Since the original observations of cerebral disconnection in experimental conditions with animal models, the surgical division of the cerebral commissures for control of seizure activity has led to the description of a human hemispheric disconnection syndrome. More recently, MR has revealed a spectrum of conditions of the forebrain responsible for the natural development and acquired occurrence of this unique commissural syndrome. Acutely, nonspecific expressions of disconnection may be observed; however, there are no explicit behavioral manifestations of this syndrome in the chronic state. The signs of stabilized hemispheric disconnection are only elicitable via sophisticated, neuropsychological testing performed by experienced technicians. Included in the present series of callosal pathology were individuals with agenesis, surgical section, infarction, trauma, glioma, and Marchiafava-Bignami disease. On specialized examination, the surgical "split-brain" patients illustrated the classic syndrome of a halt of interhemispheric transfer of information, a block of nondominant hemisphere access to language functions and an inhibition of dominant hemisphere access to superior visuospatial capacity. The nonsurgical subjects with developmental callosal agenesis and acquired pathologic processes involving the callosum revealed a varied, nonspecific reduction in cognitive function most probably related to associated extracallosal hemispheric pathology. No external abnormalities in behavior which could be attributed specifically to the callosal findings were identified chronically, although some progressive diseases suggested the presence of a subacute symptomatic phase of the disconnection syndrome. Any condition which causes the division, destructive insult, or primary ontogenic nondecussation of commissural axons will produce an interference in interhemispheric, interneuronal communication. This is manifested by individual degrees of covert interhemispheric data transfer arrest and of reduced bidirectional dominant-nondominant hemispheric access and telencephalic integrative capacity, consistent with a cerebral commissuropathy. This study reveals the MR equivalents of the hemispheric disconnection syndrome which occur in many nonspecific pathologic conditions of the cerebrum, but which result in few if any overt behavioral aberrations in the stabilized state.